Index of Fictional Creatures

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.

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. See also Our Monsters Are Different.