Clone Stan Marsh was a science project for Terrance Mephesto, but it went horribly wrong, as it broke free from Mephesto's ranch and destroyed the town of South Park. Due to his appearance, the townsfolk believed the clone to be Stan Marsh. The boys found him and persuaded him to attack Shelly Marsh, who, in turn, attacks him by physically beating him. When Shelly was finished, Dr. Mephesto had caught up with the boys and promptly killed the clone.
Clone Stan Marsh has a similar appearance to Stan Marsh. He has a larger head with stringy, black hair and an arm that is longer than the other, and one buck-tooth that sticks out of his mouth.
Clone Stan Marsh: "Bah-chan, bah-chan. Bah-chuy chuy chan." from "An Elephant Makes Love to a Pig"