Greninja did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Have a Greninja with Battle Bond defeat one Pokémon during battle
Reverts back after the battle ends
Greninja (Japanese: ゲッコウガ Gekkouga) is a dual-type Water/Dark Pokémon. It evolves from Frogadier starting at level 36. It is the final form of Froakie.
Certain Greninja have the ability Battle Bond, allowing it to transform into Ash-Greninja, similar to Mega Evolution.
Greninja is a large, bipedal, frog-like Pokémon. Its body and legs are dark blue, with single large white bubbles on its legs and arms and white spots over its eyes. On the sides of each of its upper thighs is a light blue, four-sided star. Its back feet have two toes, while its front feet have three toes. Its feet are webbed, and it has a yellow chest and a yellow-and-blue face. It has eyes with white sclera, red irises, and white pupils. A large, projecting dark blue stripe runs down the middle of its head, with projecting light blue stripes in between its large, yellow ears. Its mouth is hidden behind a large, pink tongue that wraps around Greninja's neck and extends outward behind its head.
Greninja moves with the speed and grace of a ninja, using swift movements to confuse its enemies while it slices them up with throwing stars made of compressed water; these throwing stars are sharp enough to split metal. Greninja is the only known Pokémon capable of learning Water Shuriken and Mat Block.