Mew was added as a secret 151st species late in the development of Pokémon Red and Green.
Mew was created after the main game was effectively finished and was hidden in unused space within the Game Boy cartridges. It was not intended to be obtainable through ordinary gameplay, which made the creature especially mysterious to early players.
Nintendo and CoroCoro Comic promoted Mew through a 1996 “Legendary Pokémon Offer.” Twenty winners were selected from entries and had Mew uploaded to their game cartridges. The promotion generated intense interest and helped spread word of mouth about Pokémon in Japan.
Mew is often confused with Mewtwo because their names and story connections are related. Mew is the ancient, elusive species numbered 151, while Mewtwo is an artificial Pokémon created from Mew’s genetic material and is numbered 150. Their roles are also central to the first Pokémon movie.