Judging from appearance alone, it could be hard to tell apart a monster from an animal or humanoid. For a creature to be considered a monster, it must fit some specific criteria. First, and most importantly, monsters, unlike other living organisms, aren’t “born”, they appear out of clusters of corrupted magic. The magic imitates existing lifeforms and creates creatures made out of corruption that look like them.
Their appearances and intelligence levels vary greatly, but the one thing they all have in common, aside from the fact that they are composed exclusively of corrupted magic (no blood, no organs, just corruption), is their instinct to spread corruption, usually through violence.
Because their bodies aren’t made of flesh or anything else but magic, monsters cannot reproduce.