Twrch Trwyth - Historia Brittonum

Historia Brittonum

The earliest reference to the boar Trwyth occurs in a tract called De Mirabilibus Britanniae (or Mirabilia in shorthand), variously titled in English as "Wonders of Britain," etc. The Mirabilia is believed to be near-contemporaneous to the 9th-century Historia Britonum and is found appended to it in many extant manuscripts. It gives a list of marvels around Britain, one of them being the footprint left in rock by Arthur's dog Cavall (here Latinized as Cabal), made while chasing the great boar (here called Troynt):

There is another wonder in the region called Buelt. There is a heap of stones, and one stone laid on the heap having upon it the footmark of a dog. When he hunted the swine Troynt(→Troit), Cabal, which was a dog of the warrior Arthur, impressed the stone with the print of his foot, and Arthur afterwards collected a heap of stones beneath the stone in which was the print of his dog's foot, and it is called Carn Cabal. And people come and take away the stone in their hands for the space of a day and a night, and on the next day it is found on its heap.

— Lady Guest tr., notes to Kilhwch and Olwen, Mabinogion.