So we know that there are 3 races that don’t live in Totto Land: giants, Lunarians, and an unknown race commonly theorized to be Oni, people like Kaido and Yamato.

But this is essentially impossible. In Egghead, Bonney revealed that Kuma is part of a “special race”. And considering that made him special enough to be the test subject and model for Pacifistas, it’s safe to assume it’s pretty rare.

And I don’t know about you, but it’s pretty clear to me that he isn’t a giant or Lunarian, and he’s DEFINITELY not whatever Kaido is.

So, Kuma is the third race not in Totto Land. So what are Kaido and Yamato if not Oni?

Well we know that there are MASSIVE variations among the same species in One Piece. Just compare Dorry and Brogy, average Giants to Oars, an Ancient Giant. The most notable and relevant differences between them are that Oars is far larger and has horns.

Guess who else are far larger than normal people and has horns?

Kaido and Yamato

So, my explanation is that Kaido and Yamato, rather than being their own distinct species, are essentially the Ancient Giant equivalent of Humans.