Mewtwo was created as a genetically modified clone of Mew.
In the Pokémon story, scientists used genetic material from the ancient and mysterious Pokémon Mew to create Mewtwo. The result was designed to be exceptionally powerful, and Mewtwo is associated with Psychic-type abilities and telepathy.
Mewtwo debuted in the first-generation games, where it appears as an especially powerful late-game Pokémon. Its numerical placement immediately before Mew in the original Pokédex reflects the history of Mew's late addition during development, even though Mew is numbered 0151 and Mewtwo 0150.
Ditto is sometimes suggested as the answer because of its transformation ability, but the games do not identify Ditto as Mewtwo's clone. Genesect is another artificially created Pokémon, but it was introduced much later and has a separate origin. Mewtwo's cloning story became even more prominent in Pokémon: The First Movie—Mewtwo Strikes Back.