Cuchulainn's name was originally Setanta. One day, King Conchobar(Cuchulainn's uncle.) was going to the house of Culann, a smith, for dinner. Cuchulainn was playing hurley but promised to follow Conchobar as soon as he finished.
When Conchobar and the other guest arrived for dinner, he forgot to tell Culann about Cuchulainn and Culann set his guard dog out.
When Cuchulainn arrived, the dog attacked him. Cuchulainn slammed the hurley ball down the hound's throat, killing it.
Culann and the others came out. The others were relieved but Culann was furious his guard dog was killed. Cuchulainn promised to find him a replacement but in the meantime, he would guard Culann's house. Cathbad the druid announced from then on he would be called Cuchulainn "Hound of Culann."