Mew was secretly programmed as the 151st Pokémon in the original Red and Green games.
The first-generation games were designed around 150 Pokémon, ending with Mewtwo in the Pokédex. After development was completed, the team removed unused debugging data to create a small amount of extra space. Mew was then added to that space without changing the game’s planned content.
Because Mew was not normally obtainable through standard gameplay, its existence was initially unknown to most players. Nintendo later distributed Mew through special events, which helped create rumors and renewed interest in the games. Mew’s unusual status also contributed to the excitement surrounding the original Pokédex.
Mew is numbered 151, while Mewtwo is number 150. Their names and related story elements often lead players to reverse their Pokédex positions. Mewtwo is encountered as a powerful Legendary Pokémon in the original games, whereas Mew was primarily associated with distributions and special events.