Mew is number 151, the final entry of the original National Pokédex.
Mew is a Mythical Psychic-type Pokémon and was not normally encountered during a standard playthrough of the original games. Its rarity helped create the enduring phrase “Gotta Catch ’Em All,” even though obtaining every original species required special distributions or trading.
Mewtwo, number 150, is a separate Pokémon created through genetic experimentation in the series' fiction. The similar names often cause confusion, but Mew comes after Mewtwo in the Pokédex.
The original 151 entries are collectively associated with Generation I. Although later games introduced more species and new ways to obtain Mew, its position as the final original entry remains unchanged.