User blog comment:Monkey.D.Sammy/Kaido the Strongest creature in the world/@comment-6940388-20130604101904

I would say he has eaten a Mystical Zoan fruit, the creature he can then turn into would be a shapeshifter. This would also fit his Epithet '100 Beast Kaido'. I am not sure what this fruit would be called or even if such a creature exists. This is just my theory :)