Mew was offered through CoroCoro Comic’s famous 1996 lottery promotion.
Mew was a last-minute addition to the original Pokémon games and was not normally obtainable through ordinary gameplay. To publicize the mysterious Pokémon, CoroCoro Comic announced a special promotion in its May 1996 issue. Readers could enter by sending a postcard, and selected winners could have Mew added to their game cartridges.
The promotion helped turn Mew into a genuine mystery. Many players knew that a 151st Pokémon existed, but could not find it through the normal routes, gyms, or story events. That secrecy generated intense interest and word-of-mouth around the young franchise.
Mewtwo is a separate Pokémon: it is numbered 150, while Mew is number 151. Their similar names and connection in the games often cause confusion, but the CoroCoro promotion specifically centered on Mew.