Yamato seems to have his sturdy body, but doesn't have his fruit. Kaido also thought the scabbards shouldn't of been able to stab him in base form too. Which means its not just the attribute of his DF. He must be some near extinct race or something.
I can see Kaido being from a never before seen race. But, with the amount of “normal” humans in One Piece with horns, I have my doubts. It can be that just like BM, Kaido could’ve been born a naturally strong monster, and just passed that on to Yamato.
Yamato’s case can also just be being durable thanks to the amount of beatings Kaido dished out. I’ve also had the thought that Kaido can be some kind of human/mink/fish-man/giant hybrid. That would explain why he was referred to as creature, because there’s never been something like him before. The fish-man aspect could be why he doesn’t commit suicide by drowning because he can’t drown, just like Jack. That would kinda explain his “inhuman” strength. It can tie too him having connections to PH with the Numbers, and Kaido’s fruit being the basis for Momo’s.