Mew is National Pokédex number 151.
Mew closes the original National Pokédex sequence introduced by the first-generation Pokémon games. It is a Mythical Pokémon, meaning it was distributed through special events rather than being routinely encountered during a normal first playthrough.
Mew's hidden presence became a major source of rumors and excitement around the original games. Its data existed in the cartridges even though ordinary players generally could not obtain it without an official distribution or a glitch.
Mew is often confused with Mewtwo, but they are separate Pokémon. Mewtwo is numbered 150 and was designed as a genetically engineered counterpart connected to Mew. That numbering explains why Mew is the final original entry, even though Mewtwo plays a prominent role in the first animated film.