Mew was secretly added to the original Pokémon Red and Green games to keep interest alive.
Mew was not part of the publicly announced core roster at launch. Satoshi Tajiri revealed the hidden Pokémon after the games were released, believing that its secret presence would encourage rumors, discussion, and continued curiosity among players.
This made Mew different from Mewtwo, which was central to the game’s late storyline and later became the focus of the first Pokémon movie. Mew’s hidden status also helped create the long-running idea that unexplained secrets could exist inside Pokémon games.
Mew was numbered 151 in the original Pokédex, after the 150 Pokémon that players could more readily encounter and collect. Access to Mew was initially connected to special events and distributions rather than ordinary gameplay. The mystery surrounding it became one of the earliest examples of Pokémon combining game discovery, player communities, and real-world events.