I recently picked up Skerples's The Monster Overhaul and have been using it to spice up classic monsters in my AD&D 2e campaign. It's a great book, with lots of random tables, advice, and generic maps that go along with the featured monsters. Despite that, it's not everything I want out of a bestiary, but I'm yet to find any single book that is.
When I run a D&D game, my primary bestiary is always the Monster Manual for that edition of the game. I like to run the edition "as is" for a while to get a feel for it before I start tweaking things or creating new stuff, so in a similar vein, I try to run the monsters by the book for a bit as well. After a while, I develop a sense for which monsters are particularly potent or not, and which ones are fun or boring to use, and I make adjustments as needed.
But again, no bestiary has proven to be everything I want it to be. So, I like to supplement the primary text with Monster Manuals from other editions, third party products like The Monster Overhaul, folklore and medieval bestiaries, and inspiration from books, films, video games, and the like. Sometimes I just Google "[monster name]" and see if I can find anything interesting on wikis, Reddit, YouTube, and the like (as much as I may denigrate D&D Redditors and YouTubers, they are occasionally capable of producing interesting ideas).
Seeing as how I've not found a single bestiary to rule them all, it got me thinking about what exactly is everything I would want out of a bestiary. What is my ideal Monster Manual? To answer that question, I'll look at past and present Monster Manuals, what I like about them, and what I feel they're missing.
Lost Knowledge
The actual descriptions of the monsters in original D&D's Book II: Monsters and Treasure are pretty sparse, but charming and utilitarian in their presentation. The book introduces a few details which are important to me, which no longer appear in modern Monster Manuals: number appearing, % in lair, and treasure type. A few entries also have miscellaneous information like when, how many, and what types of leaders are present in a group (for example, 30 bandits will be lead by a 4th-level Fighting-Man), or whether an orc band is transporting a wagon train, how many wagons there are, and how much gold they're carrying. By comparison, modern D&D Monster Manuals have none of the above guidance, and they rarely get so specific about things like group composition or wagon train contents (for better or worse).
In 5e, four to six young red dragons might be a "balanced" encounter for a party of six 20th-level characters, but does it "make sense" in the context of the fictional world? How many dragons typically travel together? The answer can be whatever the DM wants it to be, but this wasn't always the case: OD&D tells me that one to four dragons will be encountered - two dragons will be a mated pair of adults, and any additional dragons will be very young offspring of the two.
I know, of course, that most dragons - particularly a family of dragons - will have a lair, but unless I intentionally place a dragon's lair on my map, how do I know when one is found? OD&D tells me that randomly encountered dragons are 60% likely to be in their lair. We can also intuit from the % in lair value how significant the lair is to the monster - dragons are encountered in their lair more often than not, while elementals, by comparison, never are (presumably, they don't have lairs).
Obviously, dragons hoard treasure. In 5e, treasure hoards are generated based on a monster's CR. CRs are broken into tiers (1-4, 5-10, 11-16, and 17+) and, aside from the whims of the dice, hoards within a certain tier are identical, so an ancient red dragon's treasure hoard will be comparable to and maybe indistinguishable from a Demon Lord's (Fizban's Treasury of Dragons does change this, but hopefully my point is still clear - if not, imagine that I'm using any monster other than dragons as my example). OD&D tells me how big of a hoard (if any) and what type of treasure different monsters collect. Some collect primarily coins, or gems, or art objects, or magic items. Some don't collect treasure at all. Dragons have treasure type H, which is more or less the largest type of hoard in the game.
5e has "leader-type" monsters like the goblin boss, hobgoblin warlord, and bugbear chief, but little information is provided as to how these monsters actually organize themselves into groups. Not only do I not know how many goblins typically make up a dungeon gang, wilderness scouting party, or warband - I also don't know how many goblins are typically marshalled under a boss, or in their lair, how many bosses might be present. Certain 5e books - like Volo's Guide to Monsters, Fizban's Treasury of Dragons, and Bigby Presents: Glory of the Giants - have details like these for certain monsters, and it's no surprise that these more specific monster manuals are some of my favorite - and some of the most useful - modern D&D supplements.
Now, I want to take a quick detour to the 1981 D&D Basic Set, because I think this is the first time individual monster types are given their own morale scores (OD&D had morale rules based on Chainmail, but I don't think morale scores became a part of monster "statblocks" until Basic - I'm happy to be corrected if I'm wrong). With unique morale scores for each monster, the DM is given a simple metric for determining which monsters are more or less likely to run at the first sign of trouble or fight to the death, as well as those - like most undead - which never flee, no matter what, making them especially dangerous opponents.
5e has an extremely lacking optional rule for morale which is just a DC 10 Wisdom saving throw for every monster. Aside from that, the default assumption, presumably, is that the DM simply decides if and when it makes sense for the monsters to flee. I've written before about 5e's underwhelming morale mechanics and how I handle morale in my own 5e games.
It probably sounds like I'm being willfully obtuse. It's not especially onerous for a DM to figure out on their own how many dragons should appear in a group, something as simple as a coin flip could determine whether they're encountered in their lair or not, any remotely genre-savvy DM knows that dragons have big hoards of treasure, the number of goblins under a goblin boss is however many I need for the "story", or however many makes for a "balanced" encounter, and I have a pretty good idea of which monsters are likely to flee a combat and when.
But, as someone who is imagining so much and making so many decisions already in order to prepare for a game, I appreciate the guidelines which are provided in a Monster Manual like this. They obviate the need for me to imagine or decide yet another thing for myself. One could also argue that because these details are all a matter of setting, their exclusion provides the DM more "freedom" to decide what they want for their unique world. However, the 5e Monster Manual is not setting-agnostic - every bit of flavor text assumes the DM is using the default Forgotten Realms setting. If it's fine to include those details, why not these other guidelines from Monster Manuals of old? Even if they were included, the DM is still free to change whatever they like.
Fantastic Naturalism
AD&D's Monster Manual provides a hint of what's to come in 2e's Monstrous Manual. This is where we get what I call "companion monsters", or monsters that are typically found in the lairs of other monsters. For example, 60% of goblin lairs will be protected by 5-30 huge wolves, and 20% will contain 2-12 bugbears.
Like with OD&D's detailed accounting of "leader-types", these still serve a gameplay function- in OD&D, it's important to detail the number of bandit leaders because the player characters might end up fighting them, and the same can be said of AD&D's wolf- and bugbear-filled goblin lairs.
However, AD&D is also where "Gygaxian Naturalism" explodes. That is, we start to get a lot of information that serves seemingly little or no gameplay function:
"The intention behind Gygaxian Naturalism is to paint a picture of a "real" world, which is to say, a world that exists for reasons other than purely gaming ones. The implication is that monsters have lives of their own and thus go about their business doing various things until they encounter the player characters. Exactly what they do is described by reference to game mechanics, whether it be the numbers of non-combatants in a lair or spell-like abilities that help the monster do whatever it naturally does when it's not facing off against an adventuring party."
Monsters begin to be treated like real-world animals or people. For example, a goblin lair will contain female goblins and goblin children equal to 60% and 100% of the number of male warriors, respectively. If goblins are less like monsters and more like people, it only makes sense that they would have children in their lairs.
Sometimes these creatures are given stats, like "infant giants fight as ogres", but more often than not, the assumption seems to be that they are noncombatants. They could serve other gameplay functions - maybe the player characters need to sneak through a nursery of slumbering goblin infants, or could (Evilly) occupy said nursery and use the infant hostages to draw out the goblin chief - but more often than not, as in The Keep on the Borderlands, the grisly implication is genocidal - if the players want to wipe out the goblin lair, they better be ready to butcher a bunch of goblin workers, wet nurses, and children!
This is ultimately a matter of taste. For me, if a campaign is going to feature a quest like cleaning out the Caves of Chaos, then in that campaign, monsters are going to be monsters - the goblins spawn out of the underworld fully formed, supernatural and Evil, and must be destroyed at their magical root. If instead goblins are like people, then any quest involving goblins will be much more nuanced and reflective of real-world conflicts between people - they won't be inherently Evil, and the goal of the quest will never be to go kill all of them in their home (but certainly, if the characters are Evil, that is an avenue they might pursue).
As a result of Gygaxian Naturalism, we also get nice little tidbits like 75% of griffon lairs will have one or two eggs or fledgling griffons for every two adult griffons. Fledgling griffons sell for 5,000 gold pieces and eggs sell for 2,000 gold pieces. One could argue that this is problematic in another way, in that money-hungry players (i.e., most players) are incentivized to go out and drive griffons into extinction in order to make a quick buck.
I like it though, mostly because it generates a quest hook other than "slay the griffons" (i.e., "acquire a griffon egg or fledgling griffon", which doesn't necessitate killing the griffons), and because it allows player characters to obtain young griffons which they could potentially raise and train as mounts, which is fantastic in the literal sense. It is also up to the players to decide if the coin is worth compromising their characters' morals (if they have any). In my 2e game, the party declined a quest from an Evil wizard who wanted them to acquire pegasus eggs for his sinister experiments. Later, they encountered an adventuring party who gleefully took the wizard up on this job, which provided an Evil contrast to the party's Goodness.
The 2e Monstrous Manual is one of my favorite bestiaries of all time (in no small part due to Tony DiTerlizzi's evocative and stylized illustrations). It terms of statistics, it adds little compared to 1e's Monster Manual, but for better or worse, it goes all in on Gygaxian Naturalism. If AD&D 1e is the encyclopedia of D&D in general, 2e is the edition which produced D&D's encyclopedia of monsters.
The text describing each monster is incredibly exhaustive compared to OD&D's and 1e's sparse and often ambiguous entries. The downside is that a good amount of the monsters' game mechanics are buried within paragraphs of what would now be described as "flavor text". When running 2e, I often have to make a bulleted list of notes to accompany the monsters I want to use. As a result, it's not super useful at the table (my random encounters in the wilderness often suffer from a few minutes of dead air while I skim the monster's description for relevant game information), but it's terrific if one has the leisure of reading ahead of time, as it offers an abundance of inspiration.
Modern Manuals
All of this is not to say that I don't like D&D's modern Monster Manuals. I actually quite like the way monsters are presented in 5e's Monster Manual (as usual, I am skipping 3e and 4e because I never DMed either edition, so I'm not super familiar with those entries). 5e's statblocks have most of the information a DM needs.
I like that the monsters have ability scores and skills (which I think began in 3e), which allow them to interact with the system in much the same way as players (even if I don't see why I might ever need to know how good a monster is at Persuasion or Performance). I like that monster's actions, bonus actions, reactions, and special defenses are all plainly stated.
5e has an at times bizarre and obtuse rules vernacular that requires some research to parse, but once I familiarized myself with the lingo, I found it exactingly specific and easy to run. A knock-on effect of this is that spell descriptions have been greatly simplified, meaning that spellcasting monsters are no longer a pain to run (some of this is also attributable to the fact that I play online exclusively and am not fumbling around with multiple books, which makes this more difficult).
I like the addition of legendary actions, resistances, and lair actions, which make "boss" monsters more formidable and engaging to run. Some of the more powerful monsters have incredibly lengthy statblocks as a result, but because the information is organized in a standardized way, I've never found it too difficult to use. Compare 2e's dragons to 5e's dragons and let me know which one seems easier to run out of the book.
The presentation of flavor text is streamlined, too. Monster descriptions usually have two or three bolded headings which organize the information into chunks. If one imagines a spectrum of complexity from OSE's easily digestible bullet points to OD&D and 1e's snappy blurbs to 2e's lengthy and jumbled encyclopedia entries, 5e's Monster Manual is between OD&D/1e and 2e, which I think is the sweet spot.
Still, I can't help but mourn for what's been lost since 2e, and books like The Monster Overhaul make me wonder what else is possible. Aside from just mashing books from different editions together to fill in the missing pieces, what more does a Monster Manual need?
Hooks
I don't want monsters that simply exist, but monsters which do things in the world. Them being monsters, these things are often detrimental to the citizens of Fantasyland. Monsters cause problems that need to be solved. Problems generate quest hooks which adventurers can pursue. A hook can be as simple as "the monster is killing people, so adventurers are needed to kill it", but that gets boring fast.
Alternative hooks are often implied in a monster's description, but not always. More recent monster-focused 5e books like the aforementioned Fizban's and Bigby's provide tables of adventure hooks for dragons and giants respectively. Not every monster is suited to having a bunch of quest hooks tied to it, but I would be happy to have as little as a d4 table for more versatile monsters for whom it makes sense.
Goals, Personality Traits, Ideals, Bonds, and Flaws
This information is important for roleplaying monsters - after all, they don't exist solely to be battled and killed. Monster Manuals usually provide some information about what a broad type of monster generally wants and how it behaves, but they rarely account for individual variation.
The Dungeon Master's Guide has tables for generating these details for NPCs in general, but these don't always apply easily to specific monsters. A dragon wants different things, behaves differently, and has different secrets than a giant, and the same can be said of goblins, orcs, kobolds, and the like.
I might settle for some ideas as to what different monster types might want, but I also wouldn't be mad about having one table for beholders, another for mind flayers, and another for aboleths, for example.
Lairs
This is implied or outright stated for a number of monsters, but there aren't often a variety of options presented - trolls live in troll holes, hook horrors live in cliffside caverns, and satyrs live in forest groves. I would like at least three ideas for unique places that each monster could either inhabit or be encountered in. If these options are specific to the various terrain types where the monster can be found, all the better.
I also really like the "generic" maps included in The Monster Overhaul. If I roll a random encounter in the lair of a beholder, lich, or troll, I can easily pull the book out and improvise something using a map from that book. I don't have to make something up spontaneously or go frantically Googling "beholder lair map". The 5e monster-specific books also include some generic monster lairs, which I appreciate, but I would like to see it more - generic caves, burrows, mines, dormant volcanoes, gloomy ruins, camps, shipwrecks, islands, mountain peaks, pocket planes, and so on.
Variants
This is something that's done in quite a few Monster Manuals. 2e's entry for the hydra has the standard hydra, the Lernaean hydra, the pyrohydra, and the cyrohydra. 4e I think had a variety of goblin types - for example the cutter, sharpshooter, and shaman. 5e has variants for some of its monsters as well - the deathlock, an undead warlock from Mordenkainen's Tome of Foes, has a standard version as well as variants for the Archfey, Fiend, and Great Old One warlock patrons.
It's easiest to do this with humanoids (just design them similarly to the different player character classes) and spellcasters (just change their spell list) but I don't see why we couldn't also have, for example, harpy variants - tropical bird harpies that use illusion magic instead of enchanting songs, or vulture harpies that practice necromancy, reanimating the skeletons of their victims after they've picked the bones clean. Maybe it's not a harpy anymore if its main thing isn't singing, but I don't know - D&D never seems to have a problem with bastardizing real-world mythology, which I think is fine, because D&D isn't trying to replicate real-world myths.
The Monster Overhaul is particularly good at presenting variant monsters - different types of undead dragons, more colors of "chaos frog" (slaads), and t-rexes with special attacks and abilities, for example.
Useable Materials
This element is inspired by Dungeon Meshi, which is of course a manga (and now an anime) about eating monsters. The Monster Overhaul also has some information on what different monsters taste like and what the effects of eating them are. I had a player character in my last 5e game whose whole thing was eating monsters. I like the idea, but it's not exactly why I'm bringing up Dungeon Meshi.
What really amazes me about Dungeon Meshi is how creative it is about how characters in the world make use of monsters and their materials, even if it means challenging popular conceptions about what classic D&D monsters fundamentally are (despite the fact that the original manga's author Ryoko Kui never played D&D, Dungeon Meshi is undeniably very D&D).
For example, when I saw that one episode of Dungeon Meshi was titled "Living Armor", I thought, "How in the world are they going to eat that?" Well, spoiler alert I guess, in Dungeon Meshi, living armor (or animated armor, to use D&D parlance) is actually armor inhabited by an edible mollusk creature. To give another example, mimics aren't amorphous shapechangers that imitate mundane items - they're hermit crab things that live inside of chests. The characters use golems made of earth as living vegetable gardens, and make sorbet out of ghosts. I haven't read the manga or watched all the episodes on Netflix yet, but this kind of creativity is so exciting and inspiring, and I can't wait to see what weird uses for monsters come up next.
This reminds me of bits in the AD&D Monster Manuals where unique materials are called out for certain monsters: the aforementioned griffon eggs; ankheg chitin which can be fashioned into lightweight, strong armor; manticore pelts which sell for 10,000 gold pieces, and so on. It's not quite as exciting as some of the ideas in Dungeon Meshi, but it demonstrates that those same principles can apply to D&D.
After all, if monsters are creatures that exist in the fictional world, it only makes sense that people in that world would have all sorts of uses for them. Perhaps kraken ink and cockatrice feathers can be used to pen powerful spells. Harpy vocal cords can be fashioned into magically enchanting string instruments. Griffon eyes can be used to craft magical lenses that grant enhanced eyesight. I don't see any reason why each monster couldn't have at least one or two such uses. This is especially true of those monsters that don't keep treasure, and so don't offer much of a reward when they're defeated.
My Own Private Monster Manual
In short, my ideal Monster Manual would have clean, detailed information about the mechanical abilities of monsters in my chosen system. The 5e Monster Manual statblocks are a good example as to how relatively complex monsters can be outlined in a specific, rules-oriented language that is (usually) easily parsed by someone who's proficient in it (although the language sometimes leaves something to be desired).
Monster entries would also have flavor text descriptions akin to slightly more detailed bullet points - perhaps three to four very short paragraphs that describe the monster in some sort of default setting which the DM is free to alter. The bulk of the description would instead be replaced by a series of tables with quest hooks, roleplaying prompts, and lists of lairs, monster variants, and usable materials for each monster.
Finally, the monster's combat statistics would be supplemented with statistics and information which root it firmly in the fictional world's imagined ecology - where is the monster found, how many of them form a group (in a dungeon, the wilderness, or a lair), what other monsters it associates with, how likely is it to be found in its lair, and what treasure can be found there.
That's quite an ambitious Monster Manual. I'm certainly not going to be the one to write it, and I don't begrudge the Monster Manuals we have for not being all of these things. The point I guess is that it's worth it for the DM to have a few different Monster Manuals in their collection, and to collate the parts into a more complete whole when possible.