Cerberus was a mythological guardian of the underworld, a dog with three heads. The heads barked of the past, present and future. It was an Other-worlder.
In 2366, Deanna Troi had a waking dream in which the shape-changer Proteus and a band of Other-worlders assaulted CephCom, the headquarters of Cephalic Security in the city of Verity on the planet Rampart. Proteus shape-changed into Cerberus, frightening the security officers. (TNG novel: Gulliver's Fugitives}