Mewtwo was assigned Pokédex number 150 in the original Pokémon games.
In Pokémon Red and Green, and in their international counterparts Red and Blue, Mewtwo appears immediately before Mew in the numerical Pokédex. Mew is number 151, making the two genetically connected Pokémon the final entries in the original Pokédex sequence.
Mewtwo is described as a genetically engineered Pokémon created from material associated with Mew. In the games, research notes in the ruined Pokémon Mansion explain its origins and its escape after becoming too powerful to control. The player can encounter Mewtwo late in the adventure in Cerulean Cave.
The number is sometimes misremembered because Mew is the mythical Pokémon often associated with the “last” original entry. Mewtwo’s position also reflects its late addition to the game’s development history: Mew was added near the end, after Mewtwo had already occupied the preceding index position. Mewtwo later gained two Mega Evolutions, Mega Mewtwo X and Mega Mewtwo Y.