Mew was secretly added to the original Pokémon games without Nintendo knowing at the time of their release.
Satoshi Tajiri included Mew in the game after development was effectively complete. The Pokémon was not normally obtainable through standard gameplay, and its existence was not widely advertised when Pocket Monsters Red and Green launched in Japan in 1996. This made Mew an especially powerful source of rumors among players.
Mewtwo, by contrast, was part of the ordinary endgame mythology and is National Pokédex number 150. Mew is number 151 and was intended to be a hidden discovery. The two names and their connected genetic story are often confused, but they are separate Mythical and Legendary Pokémon in the franchise’s original lore.
Rumors about Mew helped sustain interest in the games and encouraged players to exchange information. Nintendo later distributed Mew through special events, turning the hidden creature into a major promotional feature and one of the most recognizable Pokémon of the first generation.