Nemesis Prime

Nemesis Prime is an evil clone of Optimus Prime, the leader of the Autobots from the Transformers fiction.

Transformers: Armada
In the anime Transformers: Armada, Nemesis is depicted as an enigmatic sociopathic Transformer who only wants to murder everything in sight, Decepticon and Autobot alike. Wanting as much power as he can get, Nemesis copied Optimus's abilities after being created by Unicron, a malevolent god. As Nemesis solely constructed from beligerence, the Decepticon con-artist Sideways easily manipulated him into becoming his puppet, but Nemesis was eventually destroyed via the combined efforts of Optimus and Megatron.

Transformers: Prime
In the Prime canon, Nemesis was created by anti-US government organization M.E.C.H., made specifically to be identical to the all-powerful Optimus Prime. Nemesis was created when a complete scan of Optimus's structure, the analyzation of Breakdown's structure, and dealing of the traitor Starscream's energon combined, forming a malevolent robotic humanoid that can transform into a slick black trucker.

Later, Nemesis confronted an army of Autobots, but defeated them all before taking on Optimus himself. While Nemesis had the advantage as he was incapable of feeling pain, Silas, the man controlling Nemesis, was forced to vacate his pilot seat in a fist fight, rendering Nemesis immobile. Optimus then finally destroyed his evil twin.

Trivia

 * In Prime, Nemesis is voiced by both Peter Cullen and Clancy Brown, the latter being the famous voice of Hugo Strange, Zeus (God of War), and Mr. Krabs.