Cerberus is the three-headed hound of Hell: originally guardian of the gates of Hades in Greek myth, but in Dante’s Inferno set to watch over, and tear apart, the gluttons in the Third Circle. The creature appears in several stories, including the myths of Orpheus, Heracles and Aeneas. Traditionally, it's parents are the monsters Echidna and Typhon. First appearing in Hesiod's Theogony, the beast originally had fifty heads, but by the time of Virgil and Ovid's writing - sources Dante was likely more familiar with - the number had been reduced to three.
Iconographically, the tradition of depicting Cerberus as a hound (or at least having hound-like qualities) has remained largely intact since the 6th century BCE, but often forgotten are it's serpentine features. Both Ovid and Virgil describe Cerberus as having a serpent's tail, and other snakes projecting from its body; Dante takes this interpretation further, describing the monster as a 'great worm' with taloned human hands.
Virgil also characterised Cerberus as insatiably ravenous in the Aeneid, presumably why Dante associates it so closely with the sin of greed. Virgil's given methods of placating the hound to pass into the underworld vary according to his work: one is by giving it honey-soaked bread (the Aeneid, as depicted in Pinelli's illustration above), the other is the sound of Orpheus' lyre (the Georgics). In contrast, Dante has his fictional version of Virgil mollify Cerberus by temporarily filling its mouths with mud.