Tom's Father is a pineapple, but was at one point a watermelon. Originally, due to a mistake Edd made, he was a watermelon, then retconned into a pineapple. His being a pineapple would explain his son's spiky hair.
Tom's Father has appeared in two Eddisodes so far, both in flashbacks. He first appeared during a flashback in 25ft Under the Seat as a watermelon, then reappeared in his correct, pineapple form in Fun Dead. The latter reveals that he was indirectly murdered by Matt, and more directly by a shotgun-toting bear.