Mew was secretly added to Pokémon Red and Green near the end of development.
Mew is a Psychic-type Mythical Pokémon associated with the origin of many other Pokémon in the series’ fictional lore. Its data was placed in the games after much of the development work had been completed, using a small amount of free space left in the cartridge.
Mew was not normally obtainable through standard gameplay in the original releases. Nintendo and Game Freak later distributed it through special events, which helped create rumors and excitement among players who discovered references to its existence.
Mew is often confused with Mewtwo. Mewtwo is a genetically engineered Pokémon created from Mew-related research and is obtainable through ordinary gameplay in the original games. Mew, by contrast, became famous for its secret status and event-based distribution.