Mew became National Pokédex number 151, immediately after the original 150 Pokémon.
Mew is a Mythical Psychic-type Pokémon associated with the earliest games. Its unusual status made it difficult to obtain through normal gameplay, and its existence was initially kept secret from many players.
Mewtwo occupies number 150, so the pair is a frequent source of mistaken answers. Mewtwo is a laboratory-created Pokémon linked to Mew, while Mew is presented as an ancient, genetically rich species from which other Pokémon are said to descend in the series’ fictional mythology.
The original-generation list therefore ends with Mew, even though Mew was not part of the ordinary early-game capture sequence. Later generations expanded the National Pokédex with new numbered entries, beginning with Chikorita at 152.