Cerberus was the doorman of Beau Berrant's home prior to the events described in the short story "The Last Wish". He was bigger than he was fast, and that ended up costing him.
“
The porter was too huge to have the reflexes which would let him dodge or shield himself from a quick blow given by an ordinary man. He didn't even have time to blink before the witcher's blow landed. The heavy purse struck him in the temple with a metallic crash. He collapsed against the door, grabbing the frame with both hands. Geralt tore him away from it with a kick in the knee, shoved him with his shoulder and fetched him another blow with the purse. The doorman's eyes grew hazy and diverged in a comical squint, and his legs folded under him like two penknives. The witcher, seeing the strapping fellow moving, although almost unconscious, walloped him with force for the third time, right on the crown of his head.
”
— pg(s). 223, The Last Wish (UK edition)