CERBERUS

n. (Greek Mythology) terrifying three-headed dog that guards the entrance to Hades

Cerberus the mythical three-headed dog. For other uses, see Cerberus (disambiguation). In Greek mythology, Cerberus or Kerberos (Greek Κέρβερος, Kerberos, "demon of the pit") was the hound of Hades, a monstrous three-headed dog with a snake for a tail (sometimes said to have 50 or 100 heads) called a hellhound. Other hell hounds included Orthus, his two headed brother and Argos. Cerberus guarded the gate to Hades and ensured that spirits of the dead could enter, but none could exit (additionally, no living person was to come into Hades). Among his siblings are Chimera  and the Hydra. He is the offspring of Echidna and Typhon.
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Noun
1. (Greek mythology) 3-headed dog guarding the entrance to Hades; son of Typhon
(synonym) hellhound
(hypernym) mythical monster, mythical creature
(classification) Greek mythology

n. cerberus

(n.)
A monster, in the shape of a three-headed dog, guarding the entrance into the infernal regions, Hence: Any vigilant custodian or guardian, esp. if surly.
  

(n.)
A genus of East Indian serpents, allied to the pythons; the bokadam.