Mew was hidden as the 151st species in the original Pokémon games.
Mew was added late in development and was not normally obtainable through the standard story of Pokémon Red and Green. Its existence was kept secret, creating rumors among players and giving the games an enduring mystery beyond the visible 150 species.
Mew is a Psychic-type Mythical Pokémon. In the original games, its data occupied an unused space connected with debugging and development, and official distributions later allowed players to obtain it legitimately.
Mew is often confused with Mewtwo because the names and backstories are connected. In the games’ fiction, Mewtwo was created through scientific experimentation involving Mew. Mew is National Pokédex number 151, while Mewtwo is number 150.