Satoshi Tajiri revealed Mew as a hidden Pokémon in the original games.
Mew was not part of the ordinary 150-Pokémon roster that players could obtain through standard game progression. It was secretly added as the 151st Pokémon after development, occupying a small amount of unused data in the Game Boy cartridges.
Nintendo did not initially publicize Mew as part of the main release campaign. Its existence became known through special distributions and events, including a 1996 CoroCoro Comic competition in Japan. The mystery helped generate rumors and encouraged continued interest in the games.
Mew is often confused with Mewtwo because their names and designs are related. Mewtwo is a separate Pokémon created as a clone of Mew and plays a major role in the first Pokémon film. In the original Pokédex order, Mew is No. 151 and Mewtwo is No. 150.