Which Pokémon was cloned to create Mewtwo?

Mew was cloned to create Mewtwo.

In Pokémon lore, scientists used genetic material from the Mythical Pokémon Mew to create Mewtwo. The result was an engineered Pokémon designed to be extraordinarily powerful. Mewtwo's origin is revealed through records and story events in the first-generation games and becomes a major part of the first Pokémon movie.

Mew and Mewtwo are related, but they are separate species and have different roles. Mew is portrayed as ancient and mysterious, while Mewtwo is an artificial creation whose identity is shaped by experimentation and conflict. Mewtwo's name also signals its connection to Mew without making it a simple evolution.

Ditto is sometimes suggested as an answer because it can transform and has unusual genetic properties, but the established story specifically identifies Mew as the source of Mewtwo's genetic material. Genesect is another artificial Pokémon, but it was created from a fossilized ancient Pokémon, not Mew.