Kaido showed amazing physical prowess and durability. However his devil fruit power is a mystery as of now. Here is my speculation regarding his devil fruit power. But first I will discuss why Kaido has a devil fruit power and what type it is.
Kaido is the captain of the Beasts Pirates. His pirate crew entirely consists of Zoan type devil fruit users.
It has been mentioned in One Piece manga that Kaido’s dream is to create a pirate crew entirely made of Zoan type devil fruit users. He went out of his way to fund for artificial Zoan type devil fruits. So it’s safe to assume that Kaido is a devil fruit user and the evil fruit he has is of Zoan type considering his preference.
What type of Zoan devil fruit he has? Kaido is a Yonko of the sea, one of the four strongest pirates in the world. This title shows that his devil fruit will have a strong ability. So it can be said that he has a Mythical Zoan type devil fruit.
Now comes the million dollar question. What could be his devil fruit? What creature is his ability? Here is my answer. Kaido is the holder of Ryu Ryu no Mi or Dragon Dragon Fruit. Here are my reasons for this guess.
First, let’s look at Kaido’s name. He is called Hyakuju no Kaido. According to Viz translation it means Kaido, The King of the Beasts. He is called “The Strongest Creature in the World”. Next we look at his attire.
He is wearing a rope which signifies japanese culture. Also Kaido arc is set Wano Country which is clearly inspired by japanese culture, in particular by Feudal Japan. Its people, dialect, architecture, culture and general atmosphere heavily support this.
“King of the Beasts”+”Strongest Creature”+”Japanese Culture” = Dragon. According to Japanese mythology a dragon is considered the strongest creature. It is like a Chinese dragon with long serpentine body with hard scales and claws sometimes with horns. There is also mention of dragon king according to Chinese and Japanese culture. The dragon king is the king of the dragons and he also controls all of the creatures in the sea. So it is one reason that points out of Kaido having Dragon Devil Fruit power.
Second, let’s take a look at Kaido’s ability and legend. He has been caught and tortured 18 times, and people as powerful as the Marines and other Yonko have attempted to execute him 40 times only to fail as Kaido survived every single attempt because every single execution weapon broke when being used on him. Most notably, Kaido has jumped from 10,000 meters from the sky onto the ground, creating a massive shockwave strong enough to sink a large ship nearby, and emerged only with a headache; it has even become a hobby for him to attempt suicide, only for it to fail.
So it points out that Kaido has unusual amount of defensive power. In Japanese mythology dragon’s scales are considered one of the hardest material and nearly impervious to nearly all impact and weapons. A further clue can be the tattoo on Kaido’s left arm which resembles the scales of a dragon.
*Theory by yatagarasu