Mew is National Pokédex number 0151.
It is the final entry in the original Generation I National Pokédex, which contains 151 species introduced in the first Pokémon games. Mew is a Psychic-type Mythical Pokémon and was treated as an exceptionally rare secret of the early franchise.
Mew is often confused with Mewtwo because the two names are closely related and their designs are connected in the series’ mythology. Mewtwo is a separate Pokémon and occupies number 0150, immediately before Mew. Mew is number 0151, making it the last of the original sequence.
Although Mew belongs to the original generation, it was not normally obtainable through standard story progression in the first games. Special distributions and events made it available to players. That unusual status helped create the long-running mystery surrounding the final Pokédex entry and made Mew central to early Pokémon collecting culture.