Mew was secretly added to the original Pokémon Red and Green game data.
Mew was not part of the publicly listed Pokémon obtainable through normal gameplay at launch. Its hidden inclusion became one of the most famous secrets in the early franchise and helped fuel rumors among players.
Satoshi Tajiri revealed the extra Pokémon to create renewed interest in the games. Mew’s data occupied a small amount of space that became available after development, and the creature was later distributed through special events rather than ordinary encounters.
Mew is often confused with Mewtwo because the names are related and Mewtwo appears prominently in the first Pokémon movie. Mewtwo, however, is a separate Pokémon that can be encountered in the original games after the main story. MissingNo. is also different: it is a glitch-related result, not an official species.