Mew was secretly hidden in the original Pokémon Red and Green game code.
Mew was numbered 151, just beyond the 150 species normally encountered through standard gameplay. Shigeki Morimoto added the Pokémon as an internal surprise after development, and the development team did not initially intend for ordinary players to discover it.
Nintendo later used Mew in promotional events, turning the hidden creature into a powerful source of excitement and rumor. Its unusual status helped keep attention focused on the games after release and encouraged interest in trading, events, and the wider Pokémon world.
Mewtwo is a separate Pokémon numbered 150 and is directly connected to Mew in the series’ fiction. That relationship is why the two names are often confused. Unlike Mew, Mewtwo is obtainable near the end of the original game’s main adventure, while Mew was distributed through special official events.