Mew was secretly programmed into the original Pokémon Red and Green games as Pokémon number 151.
Mew was not part of the ordinary in-game sequence of captures. It was added late in development after the game’s debugging data was removed, leaving enough space for the new Pokémon. The creature’s hidden status encouraged rumors and speculation among players.
Nintendo and Game Freak later distributed Mew through special events and promotions rather than normal exploration. This made it different from Mewtwo, which can be encountered in the games after the main story.
Mew’s secrecy became a landmark part of Pokémon history. It helped sustain interest in the original games and established the tradition of rare event Pokémon. Although Mew is numbered 151, it is not the same Pokémon as Mewtwo, whose creation and story are connected to Mew in the games and first movie.