Mew was the first generation's hidden 151st Pokémon species.
Mew appears at National Pokédex number 151, after Mewtwo at number 150. Although it was included in the original Pokémon games, Mew was not normally obtainable through standard gameplay in the initial releases. It was distributed through special events and promotions in Japan.
The Pokémon's secrecy helped generate excitement around the games. Reports of a mysterious extra Pokémon encouraged players to trade information and search for unusual discoveries. Satoshi Tajiri had included Mew late in development, and Nintendo was reportedly not aware of it at the time of the original release.
Mew is often confused with Mewtwo because their names and designs are related. Mewtwo is a major story encounter in the original games, while Mew is the hidden species that completes the first Pokédex count. MissingNo. is a programming glitch rather than an official Pokémon species, and Celebi belongs to the second generation.