CuChulainn started life known as Setanta.
When he was seven he was engaged in the favorite passtime of all boys, playing ball. This sport was called hurling and involved a long stick and a leather ball. He was suppose to be on is way to visit his uncle, Conchobar, in the neighbouring town, but like most little boys he got distracted by his game. His Uncle who was talking to the blacksmith, Culann, forgot that his nephew would be showing up and they went inside and locked the gates, first setting the watchdog out to guard.
When Setanta realized it was getting dark he hurried along and coming to the gate, started pounding on it. He did not see the dog, but heard it growl behind him, he whirled around and batted his ball at the hound lunging at his back. The ball, hit the hound in the head and he fell down dead.
The people inside had heard the pounding and the dog growling and the Uncle feared the worst. They were surpised when they opened the gate and saw a very remoseful little boy standing over the dead watch dog. Culann was heartbroken, for now who would guard his home?
Setanta, being the dutiful little person that he was vowed he would get another dog, but the smithy was certain he would never be able to again find such a highly trained animal. So Setanta vowed to him that he would remain with him and guard him and all that was his. That's how he came to be known as Cuchulainn - The Hound of Culann.
And that vow is our vow - to guard and protect all of Norrath - we are the Hounds of Culann