Mew was secretly programmed into Pokémon Red and Green as a hidden 151st species.
Game Freak added Mew late in development, after the game’s debugging data had created a small amount of available space. The Pokémon was not intended to be found through the normal story or standard in-game encounters. Its existence became a major source of rumors among players, helping sustain interest in the games after launch.
Mew is classified as a Mythical Pokémon and is the ancestor-like genetic basis for Mewtwo’s story in later Pokémon fiction. Although the original games displayed a Pokédex numbered through 151, ordinary gameplay allowed players to obtain the other Pokémon through exploration, evolution, and trading, not Mew itself.
Nintendo and Game Freak later distributed Mew through special events and promotions. This differs from Mewtwo, which can be encountered in the Cerulean Cave after the main game. The two names are related in the story, but they are separate species with different Pokédex numbers.