Mewtwo’s National Pokédex number in the original Pokémon games is 150.
Mewtwo was introduced in Pokémon Red and Blue, the international versions of the first-generation games. In the original Japanese releases, Pokémon Red and Green came first. Mewtwo appears near the end of the Kanto Pokédex and is obtainable only after the player has completed the main story and reached Cerulean Cave.
Its number is significant because Mew, numbered 151, was added late in development and placed immediately after Mewtwo. Mewtwo is therefore the final Pokémon in the original regional Pokédex sequence before the secret mythical Pokémon Mew.
Mewtwo is known as the Genetic Pokémon and is classified as a Legendary Pokémon. Its design was created by Ken Sugimori, and its story connects it to Mew through cloning and genetic experimentation. Later games added regional and expanded Pokédex systems, but National Pokédex number 150 remains Mewtwo’s identifying number.