Mew was secretly added late to the original Pokémon Red and Green games.
Mew was inserted near the end of development by programmer Shigeki Morimoto. The tiny amount of free space left in the cartridges made it possible to add the Mythical Pokémon without removing existing content.
Mew was not intended to be obtainable through normal gameplay. Nintendo and CoroCoro Comic later promoted it through a special Japanese offer, turning the hidden creature into a major source of mystery and excitement around the games.
Mew is often confused with Mewtwo, but the two have different roles. Mew is an ancient Mythical Pokémon described as having genetic links to all Pokémon, while Mewtwo is an artificially created Pokémon based on Mew’s genetic material. Mew is numbered 151 in the original Pokédex, immediately after Mewtwo at number 150.