Mew was added late to the original Pokémon games.
Mew was inserted into Pokémon Red and Green after the main development work was finished. It was not intended to be obtained through normal gameplay, making it a secret beyond the original 150 regularly documented Pokémon.
A 1996 CoroCoro Comic promotion offered a small number of players the chance to receive Mew. The promotion generated enormous interest and helped spread word of mouth about Pokémon in Japan.
Mew is often confused with Mewtwo because their names and designs are related. Mewtwo is a separate Pokémon created through genetic experimentation using Mew’s DNA. Mew is the mythical Pokémon associated with the late addition and hidden status in the first games.