Mew was a secret, last-minute addition to the original Pokémon Red and Green games.
Mew was added after the games were considered essentially complete. Its data occupied very little space, making it possible to include the mysterious species without removing major planned content. Because it was not normally obtainable during standard play, many players did not realize it existed at launch.
Nintendo and CoroCoro Comic later promoted Mew through a special “Legendary Pokémon Offer” in Japan. Twenty winners were selected from postcards and had their game cartridges modified to receive the Pokémon. This unusual distribution helped create intense curiosity and strengthened Pokémon’s word-of-mouth appeal.
Mew is numbered 151, after the original 150 species commonly encountered in the games. Mewtwo is a separate Pokémon numbered 150 and is not the same creature. The two are connected in the series’ fiction, but only Mew was the secret late addition to the original games.