Mewtwo is the final ordinarily obtainable Pokémon species in Pokémon Red and Blue.
After defeating the Elite Four and the rival, players can enter Cerulean Cave and encounter Mewtwo. It is National Pokédex number 150 and is presented as an exceptionally powerful Legendary Pokémon.
Mewtwo's backstory describes it as the result of genetic engineering based on Mew. Research documents in the Pokémon Mansion on Cinnabar Island explain that scientists experimented on Mewtwo before it escaped and destroyed the laboratory.
Mew is National Pokédex number 151, but it was not normally obtainable through standard gameplay in the original games. It was distributed through special events, so Mewtwo is the last regular in-game catch rather than the final number in the Pokédex. This distinction explains a frequent mix-up between Mew and Mewtwo.