This isn’t a fusion. It’s more like a Diancie/Carbink situation. Here’s the tale.
Few know that there was a time long ago when wishiwashi had no school form. They lived wretched lives fleeing in fear at the sight of anything that moved.
But legend has it that one single wishiwashi looked up high above the waves and became entranced by the starlit sky. For a brief moment that Pokemon forgot its fears and felt hopeful. Brave, even. So much so that it wished on the stars to share its bravery with the rest of its kind.
This is when one of the stars came down from the heavens and granted his wish.
Jirachi transformed the lone wishiwashi into a brand new pokemon altogether. It became Wishyrachi, the constellation Pokemon.
Wishyrachi is filled with the light of two dozen stars. It swims through the heavens as a singular celestial fish during the day. At night it bursts into its constellation form, taking the shape of various characters in an attempt to communicate with the wishiwashi of the world, letting them know that they are never alone. It is the work of this singular mythical Wishyrachi that inspired Wishiwashi to band together and become one of the most formidable water-types in the ocean.
by Wysardeofawes