One of the boar Twrch Trwyth’s warriors, who had been turned into a piglet. Grugyn and Twrch Trwyth’s other piglets fought against Arthur and his warriors, killing many men at the battle of Dyffryn Llwchwr. Grugyn separated from Twrch Trwyth and made a final stand at Garth Grugyn (a hill so called in his honor). He was killed after slaying Rhuddvyw and many of Arthur’s other warriors.