At the beginning of Book 1, Cerberus leaps on the kitchen table and tries to eat the food, waking up Grumio, who was sleeping on the floor. Grumio shouts at the dog, and Cerberus leaves the kitchen.
At the end of Book 1, Cerberus is found in the house during the eruption of Mt. Vesuvius sitting with Caecilius as he dies alongside his master.
Cerberus is named after the hellhound from Greek Mythology. In Greek mythology, Cerberus was a three-headed dog owned by Hades and Persephone, known as Pluto and Proserpina in Roman mythology. Cerberus is Greek for spotted, meaning the guardian of the underworld is named Spot. Good name.