Hugo Simpson II

Hugo Simpson is Bart's siamese twin (voiced by Nancy Cartwright) who appeared in the segment of The Thing and I in Treehouse of Horror VII. Since he appeared in a Treehouse of Horror he is a non-canon character.

Role in The Simpsons
In the "Treehouse of Horror VII" episode, Bart discovers he was separated at birth from a Siamese twin by Dr. Hibbert. His "evil twin," Hugo has been consigned to living in the attic of the Simpson home and is given a bucket of fish heads to eat once a week. He also does experimental surgery to prepare for his eventual reattachement to Bart, and created a "Pigeon-Rat" in the process. They discovered at the end however, that Bart was the evil twin.

Reality
However, since Treehouse of Horror episodes are non canon, Hugo is not considered an actual member of the Simpson family.

Video Games
In the Video Game "The Simpsons: Hit and Run", Hugo is one of the character costumes for Bart.