Mewtwo is National Pokédex number 150, immediately before Mew at number 151.
Mewtwo is a genetically engineered Pokémon created from genetic material associated with Mew. In the original games, it is found in the Cerulean Cave after the player becomes Pokémon Champion, making it one of the most powerful and difficult encounters in the first generation.
Its numbering is easy to reverse because Mew is the mythical Pokémon connected to its origin story and is listed directly after it. The National Pokédex order, however, places Mewtwo at 150 and Mew at 151.
Mewtwo also became central to the first Pokémon feature film, Pokémon: The First Movie, where its creation, identity, and relationship to humanity form the story’s core. Later games introduced Mega Mewtwo X and Mega Mewtwo Y, giving the species two Mega Evolutions.