Many speculating about Kaido’s real identity but I think he is a real dragon, Why? below are the subtle evidences:
1.When Law reveal which Yonko is their target, Law introduced him as Kaido “The Beast”, then Nami surprised said “Is he even a human?” (Punk Hazard)
2.Big Mom addressed Kaido as ‘thing’, somewhat referring to a non-human or maybe a mythical creature
3.Kaido named as ‘strongest creature’, which a very straight forward and on the latter Oda created more confusions to build Kaido’s character to speculate between human or creature origin.
4.When a pirate captain commonly builds a crew, he assembles it inline towards his personality, attitude or likings, since Kaido might know already he cannot get all Zoan devil fruits which somewhat limited, he contracted genius but dumb scientist like Ceasar Clown to formulate and Doflamingo as man-power to manufactured it artificially for him, it makes sense to form a crew members filled with different creatures like him.
5.Kaido is already a Yonko with plenty of resources, why still he choose to live and stay on a cave-like-castle at Onigashima?
6.Until now Oda doesn’t label Kaido as Dragon Mythical Zoan, that is pretty sneaky. Kaido is kinda weak being in dragon which that make sense, remember what Luffy says? “He transformed, He Bulked up!”, what?? Bulked up? Does Luffy mean he gets even stronger instead of getting weak after his vicious attack?
7.Moriah have a deep root anger towards Kaido and wants to get his revenge someday, if we may recall he created special type zombie(Oars) somewhat kinda features of Kaido, probably Moriah knows that Kaido being on human(Ogre, maybe?) form is much stronger, otherwise if Kaido is a Dragon Mythical Zoan which could give him a power-up, then Moriah would create special zombie type dragon instead.
8.Kaido loves being in human form (Ogre Ogre fruit?) because he loves to hold his sake pretty well
*Theory by theorymaru