As a Pokémon expert, I can tell you that Greninja does not have a Mega Evolution in the main series games. Mega Evolution is a temporary evolution that certain Pokémon can undergo in battle, but Greninja was not included in the list of Pokémon that can Mega Evolve in Pokémon X, Y, Omega Ruby, and Alpha Sapphire, where this feature was introduced.

However, Greninja does have an alternate form called Ash-Greninja, which is a special form that Greninja takes when it bonds deeply with its trainer, Ash Ketchum. Ash-Greninja is not a Mega Evolution but rather a unique form that has different battle stats and appearance. This form was introduced in the game Pokémon Sun and Moon and can be obtained through a special event distribution.

To summarize, while Greninja cannot Mega Evolve, it does have a special form called Ash-Greninja that is available in certain Pokémon games.

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Ash-Greninja. Shortly after evolving into Greninja, through its strong bond with Ash, he can call upon a transformation similar to Mega Evolution, where it takes on an appearance resembling its Trainer while being covered in a water veil. ... In its completed form, it can rival the power of Mega Evolved Pok��mon.read more >>