The description of demihuman
is misleading: "Characters who closely resemble, but are not, humans. Examples include elves, dwarves and kemonomimi." Kemonomimi is a body type, not a species.
The description of alien
is also misleading: "Characters who are distinctly non-humanoid from places other than Earth". Why other places than earth? Not all non-humanoid species on earth are animals, since there are also fantastic creatures.
I thought about it a bit, it seems the body-type defines how the animal and the human body are combined while the species defines which animal it is. For those species which are not human-animal combinations, there are the group species: demihuman and alien.
Those categories, though, by their description don't include everything. Zeus looks exactly like a human but he is no human, same goes for Superman, a zombie, an android. A slime girl, a pixie are from earth but are non-humanoid. There is no species for them. That's why
Demihuman should be: "All humanoid species which are no humans or anthropomorphic animals."
Alien should be: "All non-humanoid species which are not animals or plants."