38 years ago: God Valley
Kaido was an apprentice on the ship of Rocks D. Xebec. I think at this time he didn’t have a dragon devil fruit. He was just a normal physically strong guy.
I think he looked up to Rocks. I believe Rocks Pirates had the same kind of system that Beasts Pirates now have. If you beat someone in a higher rank than you, you can take their place.
Sengoku said infighting was common in the Rocks Pirates. This could be one of the reasons. Since Kaido was an apprentice on such a ship, he could’ve adopted the captain’s way of being a pirate.
During the incident of God Valley Rocks was defeated and most probably killed. I think during this, Rocks could’ve been betrayed by people like Big Mom, Shiki and Whitebeard. This could be the reason Kaido said “Alliances with pirates often end in betrayal.” to Kinemon and the Scabbards a few chapters ago.
I think after Rocks fell, Kaido challenged the Marines on his own to avenge Rocks and was captured or was captured during the incident itself.
Kaido is half ancient giant
This could explain the horns and his body being super sturdy. Kaido is half ancient giant, which was one of the main reasons for his capture. The World Government is kind of obsessed with Ancient Giants. They made experiments to create Ancient Giants causing the creation of the Numbers.
Oars Jr’s body went missing after the Marineford war. The reason for this is another theory of mine which says Joy Boy was an ancient giant. But you get the point. The World Government really likes Ancient Giants.
So when they saw Kaido, they seized the chance and captured him.
The Dragon Devil Fruit
I think Kaido was experimented upon (I’m not sure but maybe they extracted the Ancient Giant DNA from his to create the Numbers). I think they could’ve also fed him a Dragon devil fruit and then used him to create smiles.
SMILEs
I think the way SMILEs are made is from cells of the original animals. So they had Kaido transform into a dragon, and took some of his dragon cells to create the Dragon SMILE that Momonosuke ate.
