Mew was secretly added to the original Pokémon Red and Green games as an extra discovery.
Mew was not part of the publicly expected roster when the games launched. Its data was placed in the games after development, and the hidden Pokémon could not normally be obtained through standard play in the original versions.
Creator Satoshi Tajiri later explained that revealing an extra Pokémon helped create rumors and myths around the games. The mystery encouraged players to trade information and investigate whether the stories about a hidden 151st Pokémon were true.
Mew is often confused with Mewtwo because their names and mythology are connected. Mewtwo is numbered 150 and is obtained as a powerful engineered Pokémon in the games, while Mew is number 151 and was distributed through special events and related promotions.