Mewtwo is number 150 in the original Pokédex of Pokémon Red and Blue.
Mewtwo is a powerful Psychic-type Pokémon created through genetic experimentation based on Mew. In the original games, it appears in Cerulean Cave after the player becomes the Pokémon League Champion and completes the required story conditions. Its location and strength made it a prestigious late-game encounter.
Mew is number 151, so the two Pokémon are adjacent at the end of the original Pokédex. Mew was not normally obtainable through standard gameplay, while Mewtwo could be encountered in the game world. This distinction sometimes causes confusion when players refer to both as legendary or rare Pokémon.
Mewtwo also became the central character of Pokémon: The First Movie, whose Japanese title translates as Mewtwo Strikes Back. Later games and media expanded Mewtwo’s role, but its original Pokédex number has remained 150.