Mew was added to Pokémon Red and Green as an internal prank.
Programmer Shigeki Morimoto secretly inserted Mew after the games’ development was effectively complete. The hidden Pokémon occupied a small amount of remaining cartridge space and was not intended to be obtainable through ordinary gameplay.
Mew was initially unknown to most players, but Nintendo later distributed it through promotional events. Its existence helped generate rumors and sustained interest in the games, eventually turning the hidden creature into a major part of Pokémon lore.
Mew is National Pokédex number 151, while Mewtwo is number 150 and is a separate Pokémon created through genetic experimentation in the story. That distinction explains why early players often remembered the original games as having 150 normally documented Pokémon, despite the series recognizing 151 species.