The Sacred Five
Everything about their origin
On the planet Kaskada, a group of sacred koi swam to the top of the highest waterfall, where they were bestowed with special powers. It is there they decided to travel to our world to begin a new era.
In this modern era, koins can be reproduced multiple times. But back in their time, in order to procreate for the first time, the Sacred Five performed a very special (and irreproducible) ritual that allowed them to replicate, creating perfect copies of themselves, albeit of the opposite sex (at least that's how they explained it to us humans in a way we can understand.)