Mewthree is a fake Pokémon that commonly appears in fake cheats and fan-fictions.
Rumors of Mewthree's existence within the Pokémon games have circulated since the first generation. Many fake methods were created, that were often impossible to perform or resulted in the player deleting their save file. 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's appearance varies greatly. One version depicts him with enlarged limbs and a tuff of hair. A different version depicts him in his shiny colors with wings. Another version is the Mewtwo in armor from Mewtwo Strikes Back, although this is almost never used. No official or set appearance has been given, although the "shiny" Mewthree seems to be the most common.
Mewthree X has a set appearance, however. He seems to have enlarged limbs and four "ears" protruding to the ground. He also has red arms and a grey and purple color scheme.
It is assumed that Mewthree is stronger than Mewtwo, and similar to Mew, can learn all the abilities in the game. He is a Psychic type Pokémon.