Mew was secretly added to Pokémon Red and Green shortly before their 1996 release.
Mew was a last-minute addition to the original games and was not intended to be obtained through normal gameplay. Its hidden presence helped create rumors and mystery around the cartridges, encouraging players to keep discussing and exploring the games.
After the games launched, Mew became the focus of a promotional lottery announced in CoroCoro Comic. Twenty winners were selected and had Mew added to their game cartridges, making the Pokémon unusually rare among early players.
Mew is Pokédex number 151, while Mewtwo is number 150 and is a separate Pokémon. This numbering is a common source of confusion. Mew's unusual distribution also helped establish the tradition of special-event Pokémon in later releases.