Dark Captain America is a clone created by Dr. Doom and a boss in Marvel: Ultimate Alliance.
Marvel: Ultimate Alliance Story[]
At some point after Doom stole Odin's powers and made the heroes into his dark minions, he made a clone of Captain America after he grew impatient trying to clone the real Captain America.
After the Ultimate Alliance make it inside Castle Doom, they encounter Colossus (who is now a dark minion of Dr. Doom) and Dark Captain America where he was created to be Cap's exact double and they try to defeat them, but fails and was presumably destroyed after Doom lost his powers and face punishment to Odin.