Mew was secretly added to Pokémon Red and Green as an extra species.
The original games were designed around 150 obtainable Pokémon, but programmer Shigeki Morimoto inserted Mew late in development as an internal surprise. Its data occupied a small amount of unused space and was not intended to be obtained through ordinary gameplay.
Mew became the mysterious 151st Pokémon after Nintendo and CoroCoro Comic promoted a distribution event in Japan. A lottery selected 20 winners, whose game cartridges were sent in so Mew could be uploaded. The promotion generated intense rumors and renewed interest in the games.
Mew is often confused with Mewtwo, which is a separate Pokémon created through genetic experimentation and featured prominently in the first movie. Mew’s hidden status also differs from the regular game progression, where the first-generation Pokédex traditionally ends with Mewtwo at number 150.