The hunting of the great Twrch Trwyth (the "boar Trwyth") features in the 11thC* Welsh tale "Culhwch (pronounced Kil-hoo-crh) and Olwen"1,2. In the tale (Summary), Culhwch, the nephew of Arthur, is cursed into loving the beautiful but unattainable daughter of the giant Ysbaddaden (pronounced Usbathaden), who will die when his daughter leaves. The giant sets him a ridiculous set of Wonders to gain in preparation for the wedding, and Culhwch and Arthur's men set out to find them. One of the wonders is the comb and shears that sit between the ears of the Twrch Trwyth, a king who has been turned into a giant boar, so the giant may be beautified for the wedding.
To do this, they need to rescue the huntsman Mabon from emprisonment in Gloucester, which they do with the help of the oldest creature in Britain - a Salmon3 who may live in Lake Liuan. The Salmon carries two of Arthur's men, Kei ("Kay") and Bedwyr ("Bedevere") into the prison, and they sneak Mabon out. They then chase after Twrch Trwyth in a bloody hunt in which vast numbers of men and piglets die (Route). Eventually they grab the items, and run the Twrch Trwyth into the Severn and out of Wales, probably between Lake Liuan and the mouth of the mouth of the Wye.
After all the items have been collected, Culhwch returns to the giant for his prize, and Arthur's men "beautify" Ysbaddaden by shaving him to the bone, ears and all, before lopping his head off. Everyone else lives happily ever after.