Mewtwo is Pokémon number 150 in the original Pokédex.
Mewtwo is a genetically engineered Pokémon created from genetic material associated with the Mythical Pokémon Mew. In the original games, it is encountered late in the story in Cerulean Cave after the player becomes Pokémon League Champion.
Mewtwo’s Pokédex number places it immediately before Mew, which is number 151. That ordering helped fuel early speculation that Mew was hidden in the games, even though it was distributed separately and was not normally obtainable through standard gameplay.
The species became central to Pokémon: The First Movie, where Mewtwo questions its origins and purpose. Mewtwo is classified as a Psychic-type Pokémon and later received Mega Evolutions in Pokémon X and Y.