19. "The Thing And I" (Treehouse Of Horror VII, 1996)

As one of the most successful Treehouse Of Horror stories not to be directly based off of a previous book, movie, or TV show, "The Thing And I" is a testament to the brilliance of The Simpsons during its peak. The plot deals with Bart discovering that he has an evil twin named Hugo who Homer and Marge have kept locked up in the attic since his birth. 

Between Hugo’s “Pigeon-rat” and Homer attempting justify the need to only feed him a bucket fish heads once a week, "The Thing And I" is a hilarious story that succeeds without resorting to parody.