Still a young boy known as Setanta at the Ulster court of his uncle King Conchobar, the future Cuchulainn overhears the druid Cathbad prophesy that whoever takes up arms on that particular day will have a short life but everlasting fame; rather than being deterred, the boy insists on being armed immediately, breaking every practice weapon given to him until Conchobar's own arms are brought out, which alone survive his strength. Soon after, invited late to a feast at the smith Culann's house, Setanta arrives to find the household's fierce guard hound loosed for the night and forced to kill it in self-defense, an act that leaves Culann without protection for his home and livelihood.
Recognizing what he has cost the smith, the boy offers to take the dog's place himself until a replacement can be raised and trained, guarding Culann's house and herds personally, and from this offer takes the name Cu Chulainn, the Hound of Culann, replacing the name Setanta for the rest of the cycle.
Some versions of the tale specify that the boy killed the hound by driving a sliotar, the ball used in the game of hurling, down its throat with his hurley stick when it charged him, rather than overpowering it by main strength alone, a detail later tradition uses to link Cuchulainn's prowess in war directly to his skill at hurling, a sport closely associated with Ulster heroism throughout the wider cycle.