In Chapter 1013, Kaido defeats Luffy for the 2nd time and says “Looks like you couldn’t be Joyboy either.” Either? This implies to me that Kaido thought someone else was Joyboy before, but that hope was dismissed.
When Kaido saves King in Chapter 1035, he tells him that only HE (Kaido) has the power to change the world. I don’t believe that was arrogance. Kaido seemed to believe he was Joy Boy or had the potential to be Joy Boy.
But something happened after that and proved he wasn’t Joyboy. Or better yet, maybe some events proved to him that Oni people can’t be Joyboy or he misinterpreted something like that.
In the Marineford Arc, we know that Kaido was on his way to Marineford with King to fight Whitebeard. But what if it wasn’t to fight Whitebeard? What if it was to save Ace?
Kaido was always waiting for Joyboy in Wano. Ace arrived at Wano and left, Kaido just missed him. He hears Ace wanted to kill him. Being a strong super rookie pirate with the D initial, Kaido assumes Ace is Joyboy. He was waiting for his return to Wano.
Kaido left Wano after hearing about Ace’s execution at Marineford, not to attack Whitebeard BUT to help him prevent Ace from dying at Marineford. King also is waiting for Joyboy and accompanies Kaido. Oda makes note that King was there specifically with Kaido when meeting Shanks. Both are interested in Joyboy.
Shanks intercepts them and he tells Kaido that Ace isn’t Joyboy and that Joyboy will arrive soon. Kaido and King depart back to Wano without conflict. It explains why the Red Hair Pirates came so quickly to Marineford bearing no sign of battle. If you fight with Kaido, even as a Yonko, you should look a little bit messed up.
Theory by zyh0 (https://www.reddit.com/user/zyh0)