He's a dude (directly created by the goddess Aruru) who lived in the wild, until he is cast out from the wild because he slept with a sacred prostitute who Gilgamesh sent out so that Enkidu would stop destroying traps set by a hunter. He has emphatically beastial qualities and some argue that some art represents him with actual animal features, but in the Epic of Gilgamesh he is pretty clearly a human, and the reason he is a "wild man" is to contrast with Gilgamesh, the demigod king of a great city, as part of their rivalry/friendship thing.

I don't think those categories were quite so clearly delineated at the time

In addition to what purpleworm and buckykat said, different versions of the epic portray enkidu differently, generally getting more animalistic over time (whereas in some of the earlier versions, he's a normal human herder or hunter).

Some scholars think this transformation reflects a change in the urban-state mesopotamian perception of the surrounding non-state peoples, from viewing them as humans with a different economic system towards viewing them as mystical savages