ash-greninja

An alternate form of Greninja. Similar to Mega Evolution. But it predates the eventual Mega form it got in Pokemon Legends: Z-A.

Key characteristics:

• Red stripes on the face
• Black fins at the sides of the head, resembling Ash's hair
• Body of a lighter shade of blue
• A large water shuriken on its back (Ash-Greninja throws multiple of those shurikens at its foes)
• Often seen with a straight posture and standing on one leg

Usable in the Generation 7 games via a special Greninja obtainable from the Pokemon Sun and Moon special demo that could be transferred to the full game. This Greninja had a fixed ability called Battle Bond: when it knocked out a foe and didn't win the battle from that KO, it would transform into Ash-Greninja which drastically strengthened Greninja's attacking stats and gave a slight boost to speed and made the move Water Shuriken always hit 3 times in those games.

The form was retired when Greninja was made usable again in Pokemon Scarlet And Violet with a massive change to the Battle Bond ability that dropped the previous benefits and the transformation in favor of just a one time stat boost to attacking stats and speed when making it's first battle KO likely to coincide with the fact Ash-greninja status as a anime tie in to a character no longer in the anime with the retirement of Ash has the protagonist from the anime has of Pokemon_Horizons.

This tag implicates greninja (learn more).