Mew was secretly added to the original Pokémon games as an internal surprise.
Mew was inserted into Pokémon Red and Green after the main development work, using a small amount of freed programming space. Shigeki Morimoto added it as an internal prank, and the creature was not intended to be openly revealed to players at launch.
Nintendo later distributed Mew through promotional events, turning the hidden Pokémon into a major source of rumors and player excitement. Mew is numbered 151 in the original Pokédex and is described as a Psychic-type Pokémon with genetic links to other Pokémon in the series’ lore.
Mewtwo is a different Pokémon: it is a laboratory-created clone based on Mew and serves as the central legendary figure in the first movie. That relationship explains why the two names are often confused, even though Mew was the secret addition to the games.