Where do monsters come from?
Many people won’t care. Monsters are. Perhaps they should not be (and the PCs are dedicated to fixing the situation).
If you are running a particular sort of game – or designing your own setting – asking where the diversity of monsters comes from can be useful. Popular debates in our own world give us two obvious answers: evolution and creationism.
Evolution at first appears tricky for a fantasy world. Why do dragons have six limbs, but wyverns only four? How do we explain creatures that mix bits from different animal types, such as griffons or chimera?
Creationism appears to be the obvious choice, particularly for settings in which their are active gods. One problem with this, though, is that it can be difficult to come up with reasons for gods to have created some fairly strange things. This often leads to all sorts of niche gods, which some people don’t like.
Fantasy worlds aren’t limited to these two simple choices. There are, at the very least, variations on them. Evolution requires mutation – which can be generated via magical as well as natural effects. Chaos magic zones could create new, strange strands of a species. What happens to the fetus when a pregnant lion is turned into a sparrow by a wizard? What if it is turned back? What about magical breeding experiments?
Similarly, creationism requires creation, but it need not be from a divine source. What if powerful wizards can create life? You might not want a ton of crazy creator gods in your pantheon, but ancient, insane wizards might be another story.
There are also other options. Crossbreeding may well work differently in your world than it does in the actual one. Maybe a lion and an eagle can mate, and the result is a griffon. Perhaps the laws of sympathetic magic dictate that monsters are born to natural animals under certain conditions – a chicken egg incubated by a toad might hatch a cockatrice, for example. Planar migration is another possibility – many monsters might not be native to the world.
Is there a benefit to thinking about such things?
There can be. Knowing the origins of monsters can help you to come up with some interesting plot hooks or background/historical lore for your PCs to discover. Really, though, I just enjoy figuring this sort of thing out…