Mew is numbered 151 in the original Pokédex.
The Mythical Pokémon was designed as a secret, late addition to Pokémon Red and Green. It was programmed into unused space in the cartridges and was not intended to be obtained through normal gameplay. Its rarity helped create rumors among players before official distribution methods became widely known.
In 1996, a CoroCoro Comic promotion offered a small number of players the chance to receive Mew. Nintendo and Game Freak later distributed it through special events, making the species an important early example of event-based Pokémon content.
Mewtwo is numbered 150 and is a separate species created from Mew's genetic material in the game's story. Celebi belongs to the second generation, while Eevee is an earlier Kanto Pokémon with a different Pokédex number. Mew's position as number 151 is why it is often described as the final entry of the original Pokédex.