From A Wiki of Ice and Fire
|House Casterly of Casterly Rock|
|Title||Lord of Casterly Rock|
|Cadet Branch||House Lannister|
|Founded||Age of Heroes|
According to legend, Lann the Clever tricked them out of Casterly Rock with no weapons but his wits. Thus, House Casterly was extinguished and Lann started the line of the Lannisters, which would rule the Rock from then on.
There is no information regarding House Casterly's banner or motto, nor any member of the house has been referred to by name in the novels.
Legend says the first Lord Casterly was a huntsman, Corlos, son of Caster, who lived in a village near where Lannisport eventually sprang up. Corlos slew a lion that had been troubling the village, but he spared its cubs. The gods took favor on him and showed him where to find a huge vein of gold. Soon enough the Casterlys were rich, and had fortified this mine and tunneled ever deeper. Casterly Rock became the largest castle in Westeros. 
References and Notes