Elïavrés
A knight and sorcerer who engaged in an adulterous affair with Ysave, the wife of King Caradoc of Vannes and Nantes, and niece of Arthur. He substituted animals – a bitch, a sow and a mare – in the form of women, in King Caradoc’s bed, while he took his pleasure with Caradoc’s wife. Their affair produced a son named Caradoc Shortarm (Briefbas).
After his son became one of Arthur’s knights, Elïavrés visited Arthur’s court and challenged Caradoc to a deadly Beheading Game, but stopped short of killing him. Upon discovering his true parentage, Caradoc Shortarm informed King Caradoc, who locked his wife in a tower and forced Elïavres to copulate with a bitch, a sow, and a mare, by which Elïavres fathered, respectively, Guinalot, Tortain, and Lorigal.
The adulterers conspired to kill their son with a serpent, but were unsuccessful.
Source
First Continuation of Chrétien’s Perceval | Attributed to Wauchier of Denain, c. 1200