1 Dwalin: Oomber hoolk.
1 Mordekai: Vampire.
1 Kyros: Wolf.
2 Lambert: A wolf isn’t a monster, it’s just an animal.
2 Kyros: Well so’s a stirge.
3 Lambert: No, a stirge is different... unnatural... somehow...
3 Kyros: It lives in the natural world.
4 Kyros: It’s not as if there’s some authoritative Manual that tells us what all the monsters are.

I was discussing this with some friends and we realised that in-universe, there's no really good way to tell the difference between an "animal" and a "monster" that would produce the same categories that we apply from our real world point of view.

In the real world, an "animal" is a creature that really exists, whereas a "monster" is a creature that exists only in fantasy fiction/gaming. So a wolf is an animal but a stirge is a monster. But within the fantasy world, arguably a wolf is a greater and scarier threat, so more likely to qualify as a "monster" than a stirge, which is after all just a natural creature - an animal - in the world. Where and how do you draw the line?

So if there are learned sages within the fantasy world writing bestiaries of animals and monsters, dividing up the natural world into various categories and classes of animals, they're probably not going to be the same categories that we would use in the real world.