Mew (Pokémon)
In Pokémon Red and Blue the player first learns of Mewtwo by research papers left in a ruined laboratory on Cinnabar Island. Mewtwo is told to be a mutant child of Mew made after years of work by one scientist. Mewtwo is obtainable in the Cerulean Cave, which opens after defeating the Elite Four. In the remakes Pokémon FireRed and LeafGreen the player now has to  get 60 Pokédex entries before they can go in the cave.