Mew is number 0151, the final entry in the original 151-species Pokédex.
Mew is a Psychic-type Mythical Pokémon introduced in the first generation. It was not normally obtainable through standard gameplay in the original Pokémon Red, Green, and Blue games, which helped make it a subject of rumors and extraordinary excitement among early players.
Mewtwo occupies number 0150 and is a separate Pokémon. In the games’ story, Mewtwo is connected to Mew through experiments involving genetic material, but the two Pokémon have different designs, abilities, and Pokédex entries.
The original Pokédex ended at Mew because the first generation contained 151 officially recognized species. Later generations expanded the National Pokédex with new numbers, so Mew remains the final entry of Generation I rather than the final Pokémon overall.