Theory Poseidon was the previous user of the Seiryu fruit

[Monkey D Theories]

The seiryu fruit is a model of the fish fish fruit. It is based off the myth of a fish becoming a dragon.

Spoiler

The kingdom that rules over fishman island is known as “ryugu kingdom” and it’s palace has a big dragon on it.

Given how mermaids cant walk until they are older, I think it’s likely Poseidon are the seiryu fruit so she could be active on the surface world, flying above the clouds. We do know that one of the big dreams of the fish men was to live on the surface.

I think she used this fruit to compete in the void century war and fight Imu.

Now that Kaido is dead, I think Shirahoshi could take up the fruit. This would let her go from being a weakling to becoming a powerful ally in the fight against Imu.

 

[Charlotte Horchata]

It is based off the myth of a fish becoming a dragon.

It's not, it's based on Eastern dragons being more related to fishes than reptiles, with that specific myth being just one example of such relationship. The fandom simply asumes Kaidou's fruit is a reference to that myth because their only knowledge about Japan is based on pop culture; the Pokémon Gyarados in this case.

Ryugu Palace is a real place from Japanese mythology and it was the place where the dragon god lived with his daughter, Otohime. The dragon statue has nothing to do with Poseidon eating the Seiryu fruit, but it's a direct reference to mythology.

 

[JackieDaytona3019]

This theory doesn’t pass the smell test