After the 1996 Japanese release of Pokémon Red and Green, rumors of the mysterious 151st Pokémon began circulating. With its Pokédex completed at 150 Pokémon, players had no reason to believe there could be one more -- until reported sightings of Mew, a Mythical Pokémon, became acknowledged.
Source:https://www.cbr.com/mew-secretly-programmed-pokemon-red-blue-green/