Dungeonomicon (DnD Other)/Bionomicon
From D&D Wiki
The Chicken and the Egg:
In our own world, the question of the chicken and the egg is one put out mostly to confuse the very young. It actually has a definitive answer, the egg came first. It doesn't even matter where you draw the line as to what is a chicken and what is some other creature, because wherever that line is drawn, the creature in question was first an egg and its own parents were not chickens. But in the D&D world, that's an open question because a lot of creatures are made rather than born and appear in the world fully formed. Golems simply don't need a stable pool of Golems in order to maintain genetic diversity. They don't even have genetic diversity.
Lots of other creatures in D&D reproduce in a completely magical way. Demons simply spawn out of flaming pits of rebirth, Aleaxes are created from the fact that a god got spent some divine focus (it comes somewhere between "Place Papal Magnet" and "Earthquake" I think), and chimeras are assembled out of parts by mad wizards.
Small Isolated Populations are Bad:
Bad things happen to a species if there aren't many instances of them in the world. Cheetahs can all accept skin grafts from each other and are all weak to the same diseases. Seriously, all cheetahs could go extinct next year, their existence is that fragile. In D&D there are numerous species that are less well represented in the world than cheetahs, so why don't they have the same problems?
Truth be told, some of them do. Many times a dungeon will contain a "unique monster" that the player characters will go and kill. That's an extinction event right there. There will be no more generations of "five armed fire troll" once you kill Togor the five armed fire troll. But some of them do not, and the reason is that D&D has a wealth of completely magical ways to keep a lineage from drying up. Even if a creature can't find a mate of its own kind anywhere in the world, there's always the planes and the realms of sorcery. An Archon or Devil can provide a means for any creature to create a new generation. In doing so, the new creature is sometimes pretty much indistinguishable from its mortal parent, and sometimes it shows up as a half-whatever. The point is, noone has to be doing anything particularly weird for the world to have half-fiend dire tigers in it. Which is a load off our minds, because the D&D world does have half-fiend dire tigers in it.
We Eat What We Like:
It has been noted by many an observer that actually humans are a top predator themselves, that it takes nearly 20 years for a human to grow to full size, and they're only 70 kilograms at that point. Thus, the concept of a creature persisting on a diet of human flesh is pretty much absurd. Especially if it lives in an out-of-the-way area like a mountain top or the bottom of a forgotten cave, there's just no way that something can live on manflesh alone.
Some monsters however, get the vast majority of their sustenance through magical means and only need small influxes of real food from their human food. The classic example, of course, is Vampires. They consume much less in food energy than they use up in maintaining their undead existence. Most of their energy is actually siphoned off the negative energy plane, and the drinking of human blood is just a symbolic evil act that they need to perform in order to keep that juice flowing. Similarly, mindflayers are sustained not by the nutritive value of brains, but by their own psychic powers. They need to eat the brains of intelligent creatures to keep their psychic powers sharp, and they need their psychic powers to be sharp in order to survive day to day without eating enough calories to keep themselves alive in the normal way.
Secondarily, lots of creatures have a perfectly fine diet of normal food and simply happen to attack and eat humans if they encounter them. A gelatinous cube, for example, lives just fine off of the lichens and offal that it scrapes off the walls and ceilings with its passing. But it certainly won't turn down a meal of 70 kilos of meat if it comes to that.