Kaido (King of the Beast) is known as the Strongest Creature in the World, which means he is not human, so when Kaido transforms into a Dragon, maybe that is his true form
My opinion is Kaido was a Dragon who eats Mythical Zoan-type Devil Fruit, named Hito Hito no Mi, Model: Monster Because his body is big, strong and can't die even falls from a sky island with a height of 10,000 meters, as well as Chopper who ate Devil Fruit Hito Hito no Mi's he could walk on 2 feet and could speak like a human