Mew was hidden in the original Pokémon Red and Green games as a secret discovery.
Mew was not part of the ordinary 150-Pokémon collection presented to most players. Its presence in the game code created rumors and encouraged players to investigate whether an additional Pokémon existed beyond the advertised roster.
Game director Satoshi Tajiri later explained that revealing Mew helped maintain interest in the games. Mew was eventually distributed through special events rather than obtained through a normal in-game encounter in the original releases.
Mew is often confused with Mewtwo because their names and designs are related. Mewtwo is a major story opponent and a genetically engineered Pokémon, while Mew is a Mythical Pokémon associated with the hidden secret of the first games.