Celtic Fairy Tales, by Joseph Jacobs, , at sacred-texts.com
He turned back in a rage to his castle, and as he came to the gate, who should he see but Gellert come bounding out to meet him. But when the hound came near him, the Prince was startled to see that his lips and fangs were dripping with blood. Llewelyn started back and the grey-hound crouched down at his feet as if surprised or afraid at the way his master greeted him.
Now Prince Llewelyn had a little son a year old with whom Gellert used to play, and a terrible thought crossed the Prince's mind that made him rush towards the child's nursery. And the nearer he came the more blood and disorder
he found about the rooms. He rushed into it and found the child's cradle overturned and daubed with blood.
Prince Liewelyn grew more and more terrified, and sought for his little son everywhere. He could find him nowhere but only signs of some terrible conflict in which much blood had been shed. At last he felt sure the dog had destroyed his child, and shouting to Gellert, "Monster, thou hast devoured my child," he drew out his sword and plunged it in the greyhound's side, who fell with a deep yell and still gazing in his master's eyes.
As Gellert raised his dying yell, a little child's cry answered it from beneath the cradle, and there Llewelyn found his child unharmed and just awakened from sleep. But just beside him lay the body of a great gaunt wolf all torn to pieces and covered with blood. Too late, Llewelyn
learned what had happened while he was away. Gellert had stayed behind to guard the child and had fought and slain the wolf that had tried to destroy Llewelyn's heir.
In vain was all Llewelyn's grief; he could not bring his faithful dog to life again. So he buried him outside the castle walls within sight of the great mountain of Snowdon, where every passer-by might see his grave, and raised over it a great cairn of stones. And to this day the place is called Beth Gellert, or the Grave of Gellert.