Many stories of King Arthur and the Knights of the Round Table are based on older Celtic legends. You might recognize a few elements in this tale of Sir Eliavres.
Eliavres was a magician as well as a knight and during the course of his adventures, he fell in love with King Arthur's niece, the beautiful Ysaive. Unfortunately Ysaive was already married to King Caradoc, the ruler of Vannes and Nantes. Using his magic, Eliavres made it so that whenever the royal couple started to couple, Eliavres took the king's place and substituted either a bitch, a sow, or a mare for Caradoc.
Eventually the queen gave birth to Prince Caradoc Briefbras who discovered the truth about his parentage. He told the king who understandably was somewhat upset. On threat of bloody death, King Caradoc forced Eliavres to publicly have sex with another bitch, sow, and mare. Each of these bestial encounters resulted in a pregnancy, with the bitch giving birth to a boy named Guinalot, the sow producing Tortain, and the mare Lorigal.
Again not much is said about the animal/children. I could see a movie with Steve Irwin (the Crocodile Hunter) playing Guinalot, again Jason Alexander as Tortain, and Patrick Warburton as Lorigal.
The funny thing is that many myths involve bestiality (with a god in the form of an animal) but the Welsh seem to have a knack for producing human children of animal mothers (Irish stories often leave the human/animal offspring in the form of the animal).