Mew was secretly added to the original Pokémon Red and Green games after development was nearly complete.
Satoshi Tajiri inserted Mew into unused space in the games' data after the debugging process. Nintendo staff were reportedly not aware of the Pokémon at release, and it was not part of the ordinary in-game progression. Its hidden presence helped create mystery around the games.
Mew was numbered 151, after Mewtwo at number 150. Unlike Mewtwo, which could be encountered in the game world after completing the main adventure, Mew generally required a special distribution or a particular glitch in the original releases.
Rumors about a hidden Pokémon spread rapidly among players. Nintendo later used events and promotions to distribute Mew, turning an obscure secret into a major part of Pokémon culture. Mew is often confused with Mewtwo because their names and designs are related, but Mew is the smaller, mythical ancestor in the game's story.