Satoshi Tajiri revealed Mew as a hidden Pokémon in Pokémon Red and Green to keep interest in the games alive.
Mew was not listed among the original 150 Pokémon that players could normally obtain through standard gameplay. Its presence in the game data became a closely guarded secret and encouraged rumors about whether an additional Pokémon existed beyond the expected roster.
Tajiri later explained that revealing Mew created myths and helped maintain interest in the games. Mew’s hidden status also fit the design of the original cartridges, where the Pokémon was present but not ordinarily obtainable through the normal adventure. It eventually became available through special events and distributions.
Mew is National Pokédex number 151, immediately after Mewtwo, which is number 150. The two names are sometimes confused because both are rare, powerful Psychic-type Pokémon connected to the first generation’s story. Mew is presented as an ancient ancestor of other Pokémon in the games’ mythology, while Mewtwo is an artificial clone created from Mew’s genetic material. That distinction made Mew a memorable secret at the heart of Pokémon’s first anniversary-era mythology.