Mewthree in Pokemon Chaos Black
Mewthree is a fake Pokemon encountered in Pokemon Chaos Black.
Rumors of Mewthree's existence within the Pokémon games have circulated since the first generation. These stories have led to a number of hack games focusing on this unofficial Pokémon.
The most notable of these is Pokémon Chaos Black, in which Mewthree can evolve into another fake Pokémon, Mewthree X. Mewthree was also one of the names given to the silhouette of Lucario early after its release on many internet forums because it appeared strikingly similar to Mewtwo.
Mewthree in Pokémon Amber
Mewthree is also encountered in Pokémon Amber if you make Mewtwo hold the stage three armor. After holding the armor, he will become Mewthree in battle, however, if the armor is taken off, he will revert back to Mewtwo form. This form changes him to a Phychic/Ancient type pokémon and his Sp. Attack and Sp. Defense increase immensly.
Mewthree's existence has been rumored as far back as the release of Pokemon Red and Blue.
Mewthree can evolve into Mewthree X in Chaos Black.
This Pokemon is proved to be an alternate form for Mewtwo, not a separate Pokemon.