Mew was secretly added to the original Pokémon games as a last-minute addition.
Mew was not part of the publicly announced core roster when Pocket Monsters Red and Green launched in Japan. Its data was inserted after development, using a small amount of freed space in the cartridges. The creature was intended to support a later promotional event rather than ordinary in-game discovery.
Nintendo and many players were initially unaware that Mew existed. A CoroCoro Comic promotion in 1996 offered a limited number of players the chance to receive it, creating rumors and intense interest around the games. Mew therefore became one of the earliest examples of a secret, event-distributed Pokémon.
Mew is number 151 in the National Pokédex, while Mewtwo is number 150. The similar names and their connection in the first movie often cause confusion, but Mew is the ancient, elusive species that was added secretly to the original game data.