Who secretly added Mew to the original Pokémon games as an internal prank?

Shigeki Morimoto secretly added Mew to the original Pokémon games as an internal prank.

Mew was not part of the publicly announced original 150 Pokémon during development. Morimoto added the Pokémon after the game’s debugging work, using a small amount of space that became available when development material was removed. The hidden inclusion helped create one of Pokémon’s earliest mysteries.

Players initially had no ordinary in-game method to obtain Mew. Nintendo later distributed it through promotional events, which made the elusive Pokémon a major source of rumors and interest among fans. Mew’s existence also helped explain why some players believed the games contained secrets beyond the advertised Pokédex.

The original games ultimately became associated with 151 Pokémon because Mew was numbered 151 in the National Pokédex. This is different from saying that Mew was part of the original publicly promoted roster. Mew was a hidden addition that later became an official Mythical Pokémon.