At some point in history somebody decided to make up some of their own animals and beings, and sometimes the particular creature they made up was interesting enough that others kept using the same made up creatures in their stories. Well, here they are. All of them. Well, as many as we could get.
For organisms with an intelligence about equal to humans or otherwise sapient which usually don't get wantonly slaughtered by heroes without moral repercussions, see Fantastic Sapient Species Tropes.