Just a different kind of human like Moriah or Oars
A regular animal like Bepo
An animal who eat a new Hito Hito no Mi model
A Kaiju (i.e. Godzilla)
A Vampire, Mummy or other folkloric being
Not less Human than a Klingon.
probably a mythical beast. minotaur perhaps? but with a human like face
Maybe Kaidou is a Leviathan