Since chapter 921 there have been many speculations about which devil fruit Kaido has.
While I certainly like the idea that Kaido is a dragon with the Oni Devil Fruit it still leaves the question why Oda changed Kaido’s design since his first appearance…
… unless he hasn’t!
Many people have been speculating that Oda has used the old Kaido design for Ashura Doji, who changed his name to Shutenmaru after Oden’s death.
On the other hand we have this theory that speculates that the new Kaido design is just his human-dragon hybrid form:
*Theory by Papa-Yaga