Transsexual Bestial Incest
(Man, I'll get the hits now)

According to Welsh legends, there once lived a perverted young man named Gilfaethwy who happened to be the brother of the magical Gwydion fab Dôn. Gilfaethwy lusted after the beautiful Goewin, a footholder of King Math fab Mathonwy's court. Naturally footholders were required her to be virgins so Gilfaethwy got no satisfaction.

Ever the brotherly sort, Gwydion fab Dôn used his magic to spark a war between King Math and the national hero Pryderi over his magic pigs. Gwydion fab Don visited Pryderi and asked to trade 12 stallions and 12 greyhounds with gold collars and shields for the swine. Pryderi agreed so Gwydion fab Don hurried away, knowing the horses and dogs were magical constructs and would fade away after 48 hours.

Upon learning he'd been duped, Pryderi declared war on Math and while the king at battle, the two brothers took turns raping Goewin. With the war at a stalemate, the two sides agreed to a duel of single combat between Gwydion fab Don and Pryderi. Using his magic, Gwydion easily killed the heroic Pryderi and only afterwards the king learned what the he and his brother had done.

Math used his own magic to turn the brothers into a mated pair of deer for a year. Their sex drive wasn't diminished so by the end of the term, they had borne a fawn. The next year Math turned them into a couple of pigs and they again bore a son. Finally Math turned them into wolves, producing a wolf cub at year's end. At this point, Math relented and changed them all back to humans. Gwydion fab Don continued his tricks in another cycle of legends but I could find nothing else about the children other than that the deer-boy was called Hydwn, the wolf cub Bleddyn, and piglet Hwychdwn. The Welsh never mentioned how having two transgendered brothers for parents affected the boys or if their early childhood as animals had any repercussion.

I can imagine a movie based on the exploits of the grown up animal/children, starring Ben Stiller as Bleddyn, Jason Alexander as Hwychdwn, and Keanu Reeves as Hydwn. Hilarity ensues.