It was stated that Totto Land had every race in the world except for 3.
We now know that both Kuma and King are the last members of their individual races, and that no giants live in Totto Land.
Which means that the 3 missing races are Giants, Lunarians, and Buccaneers.
Which means, most likely, Oni isn’t an actual race, and Kaido’s just really big and strong for no reason, just like some other humans, or that it’s just a variant of human, like Ancient Giants and Giants