Mew was the hidden Pokémon added to the original Pokémon Red and Green games as National Pokédex number 151.
Mew was not part of the main public roster that players normally encountered during the original adventure. Its data was placed in the games late in development, and special distributions and events later gave players ways to obtain it.
The decision helped generate rumors and discussion around the games. Mew's position after Mewtwo also created one of the franchise's best-known numbering details: Mewtwo is number 150 even though Mew is described as its genetic source.
Mew is classified as a Mythical Pokémon, whereas Mewtwo is a Legendary Pokémon. Celebi and Jirachi are also Mythical Pokémon, but they belong to later generations and are not part of the original 151.