"Cerberus, monster cruel and uncouth"

Public DomainCerberus - Credit: William Blake

(Canto 6, line 13)

Cerberus was a three-headed dog watching at the doors of the underworld in Greek mythology, the offspring of the giant Typhon and half-snake, half-woman Echidna. Virgil, Ovid and other classical authors have all described him with three heads, tail of a snake, etc. Dante follows them here, but adds to the decription.