As an expert in the field of Pokémon, I can provide you with an accurate answer regarding the Mega Evolution of Ash's Greninja.
Ash's Greninja, known as a unique bond Pokémon, does not have a Mega Evolution form in the traditional sense. Mega Evolution is a temporary evolution that some Pokémon can undergo in battle, which was introduced in the Pokémon X and Y games. However, Greninja, including Ash's Greninja, was not given a Mega Evolution in the games or the anime.
In the Pokémon anime, Ash's Greninja has a special bond with Ash that allows it to transform into a form known as "Ash-Greninja," which is distinct from Mega Evolution. This transformation was first seen in the episode "The Cave of Trials!!" and is a unique ability that sets it apart from other Greninja.
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