Mew was secretly added to the original Pokémon games to create rumors and interest.
Mew was not part of the ordinary 150-Pokémon progression in the Japanese Red and Green games. Its data was hidden in the software, and it became the 151st entry in the National Pokédex.
The secret helped fuel speculation among players, many of whom heard stories about a mysterious Pokémon that could not be obtained through normal play. Official distributions later made Mew available to players.
Mew is classified as a Mythical Pokémon, while Mewtwo is a separate species numbered 150. The two are closely connected in the fiction: Mewtwo was created from Mew’s genetic material, but Mew—not Mewtwo—was the hidden addition used to build early mystery around the games.