In Chapter 999 Big Mom revealed that on the day Rocks fell, she was the one who gave Kaido his Devil Fruit: the mythical Uo Uo no Mi (Fish-Fish Fruit). Because of that, she believes he owes her a debt for life.
Ironically, despite a series of Zoans (the Ryu Ryu no Mi) being named after dragons, and this being the only Devil Fruit seen so far that actually enables transformation into a dragon (with the fruits seen so far in the Ryu Ryu series granting dinosaur transformations), this fruit is not named after dragons but after fish.
The Uo Uo no Mi, Model: Seiryu is currently the only known Zoan fruit named after an aquatic species, although any potential underwater abilities it may grant are canceled out due to being deactivated by water.
Due to this fruit being named for fish, its transformation is possibly based on the mythical dragon-fish hybrid Chiwen, although Kaido’s transformed body more resembles a typical serpent-like Eastern dragon.
The fruit’s power could also potentially be based on the Chinese myth of the Dragon’s Gate, in which carp transform into dragons upon swimming up to the top of a great waterfall. Wano, where Kaido resides, is located atop a waterfall which giant carp swim up.
Oda left us so many hints that Kaido had the Fish-Fish Fruit all along and it’s only in retrospect that we can truly appreciate it:
– The opening to Wano is literally Koi/Carp jumping over a waterfall just like in the popular eastern legend that involves a Carp climbing a waterfall and being transformed into a dragon by the gods. Honestly surprised more of us didn’t connect the possibility right here.
