This ballad relates the story of Gelert, the dog of Prince Llywelyn ab Iorwerth. According to the tale, Llywelyn embarked on a hunting trip and left his infant son in the charge of his faithful dog Gelert. On his return, the Prince was greeted by Gelert, who noticed the dog's muzzle was soaked in blood, and his son was nowhere to be seen. Llywelyn attacked the dog, and it fell to the ground gravely injured. However, within minutes he heard a cry and discovered his son, safe in his cradle. Beside the cradle lay the body of a giant wolf covered with wounds, the result of a fight to the death with hound Gelert. Llywelyn strode back to his faithful dog and watched it die from his actions. The village of Beddgelert [lit. Gelert's grave] in Snowdonia is reputed to be the final resting place of the hound.

Source: http://www.snowdonia.org/