Cerberus

Offspring of Typhon and Echidna, Cerberus was set by Hades to guard the entrance to the underworld at the River Styx. He greeted the newly dead with a wagging tail, then prevented their return. Only three mortals ever passed him: Heracles subdued him bare-handed for the eleventh labor; Orpheus lulled him to sleep with music; the Sibyl gave him honey-cakes drugged with herbs.

A massive hound with three heads (some sources say fifty or a hundred), a serpent for a tail, and serpent heads growing from his back and neck. Each of his three heads was said to represent past, present, and future, or youth, maturity, and old age.

Prevented any shade from leaving the underworld and any living being from entering without divine sanction. His gaze induced terror and madness in mortals. His saliva, where it fell on earth during Heracles' abduction, grew the poisonous plant aconite.