Mew was the hidden Pokémon added as a development prank in Pokémon Red and Green.
Programmer Shigeki Morimoto inserted Mew late in development after space was freed from the game’s code. It was not intended to be openly obtainable through ordinary gameplay, and its presence was initially unknown to most players and even some people involved with the project.
The developers later revealed Mew as an extra Pokémon, helping generate rumors and sustained interest in the games. Nintendo distributed it through special promotional events rather than placing it in the normal Kanto progression.
Mew is often confused with MissingNo., a famous glitch-related phenomenon, but they are different. Mew is an official Pokémon species, while MissingNo. is an unintended data artifact. Mew was also numbered 151 in the original Pokédex, after the first 150 species.