Mew was secretly added late in development to the original Pokémon games.
Game Freak programmer Shigeki Morimoto added Mew near the end of Pokémon Red and Green’s development. The developers had not planned for ordinary players to encounter it, and its hidden data occupied a small amount of space left after debugging work.
Mew’s existence became widely known through a 1996 CoroCoro Comic promotion. A limited number of readers were selected to receive Mew on their game cartridges, creating enormous curiosity around the mysterious 151st Pokémon. It could not normally be captured through the standard story in the original games.
Mew is a Psychic-type Mythical Pokémon and is presented in the fiction as an ancestor-like source of genetic material for many Pokémon. It is often confused with Mewtwo, which is a separate genetically engineered Pokémon created from Mew. That connection is central to Pokémon: The First Movie but does not make the two species identical.