Something I've never really liked in fantasy bestiaries is the trend to give monsters pseudo-naturalistic ecologies. These are monsters, not animals: even in the game rules they're labeled "monstrosities" or something.
So, to that end, I prefer to give my monsters decidedly unnatural ecologies. For example:
- Monstrosities are creations of Chaos that blight the world. Whether they wander out of the Chaos Wastes, are brought into being by the rituals of cults, summoned by the creation chambers of living dungeons, or spawned by mysterious wandering progenitors, they are blights that despoil and bring ruin to any ecology they enter. Naturally, they're despised by druids and you can always count on them to aid you in any fight against these things.
- Hydras don't reproduce on their own: new hydras are created by fighting hydras (hence the "hydra effect"). Whenever a hydra's head is cut off, it doesn't die: it tries to flee as best it can. The head may even sprout legs, wings, fins or other appendages that it retains as it grows into a new hydra, thereby explaining the diversity of hydras. Each generation is smaller than the last until heads simply die after being severed, suggesting that an original Hydra exists somewhere.
- Basilisks and cockatrices are all sterile and created by alchemists, a process apparently involving multiple oddities such as a yolkless egg laid by a rooster being incubated by a toad or snake.
- Thessalhydras don't hybridize through sexual intercourse with other monsters. Rather, they eat other monsters and severed heads will grow into hybrids of whatever the last monster eaten was.
- Chimeras don't have consistent anatomy. Their anatomy constantly shifts, so that the arrangement of heads and appendages is different from day to day. Even the animal components can change over time, adding or removing heads and appendages. As a result, the monsters are in constant pain and quite temperamental.
And so on and so forth. I'm sure I've said this in other posts but it's been years so I'm saying it again.