Mew was secretly added to the original Pokémon Red and Green games as an internal prank.
Programmer Shigeki Morimoto inserted Mew after the game’s development was effectively complete. The creature occupied a small amount of unused data space and was not intended to be openly revealed to players through ordinary gameplay.
Nintendo later chose to distribute Mew through promotional events, turning the hidden character into a powerful source of rumors and excitement. Because Mew was numbered 151, it became the final entry in the original National Pokédex, following Mewtwo at number 150.
Mew is often confused with Mewtwo, but the two have different roles. Mew is a rare Mythical Pokémon said to contain the genetic code of every Pokémon, while Mewtwo is an artificial Pokémon created from Mew’s DNA. MissingNo., another famous name associated with the original games, is a glitch rather than an official species.