Mew was the secret Pokémon added to the original games and promoted through a CoroCoro lottery.
Mew was a last-minute addition to Pocket Monsters Red and Green. It was not normally obtainable through ordinary gameplay, which made its existence a mystery to many players. The Pokémon was hidden in the game data and became the focus of a promotional campaign in CoroCoro Comic.
In 1996, readers could enter a lottery by sending in a postcard. Twenty winners were selected, and their cartridges were returned with Mew uploaded onto them. The promotion generated rumors and conversation around the games, helping sustain interest after release.
Mew is distinct from Mewtwo. Mewtwo is a laboratory-created Pokémon connected to Mew’s genetic material and plays a major role in the first Pokémon movie. Mew is the smaller, mythical Pokémon associated with the original hidden secret.