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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
No matter what creature Kaidou is, he was probably never a human. So far we have Chopper, Bepo, Pappug and Pekom who are all animals with the ability to speak. I think Kaidou is someone along those lines but more sinister.
Not less Human than a Klingon.
probably a mythical beast. minotaur perhaps? but with a human like face
Maybe Kaidou is a Leviathan