Cerberus
Webster's Dictionary [1]
(1): (n.) A genus of East Indian serpents, allied to the pythons; the bokadam.
(2): (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.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]
a fabulous dog in Greek mythology, was the son of Typhon and the snake Echidna, and was a hateful monster which sometimes is spoken of as having fifty, sometimes a hundred heads, but is generally represented with three. (See Pluto); (See Serapis). The poets describe him as snake- haired, with a dragon's tail, of frightful barbarity, poisonous breath and deadly sting lie guarded the shades of the infernal regions, allowed all to descend, but none to return, and the severest work was to fight this monster. Various persons attempted to combat him; among them, Orpheus, with his lyre with which he put him to sleep. Hercules also was ordered, as one of his twelve works, to bring Cerberus from the infernal regions.
The Nuttall Encyclopedia [3]
The three-headed or three-throated monster that guarded the entrance to the nether world of Pluto, could be soothed by music, and tempted by honey, only Hercules overcame him by sheer strength, dragging him by neck and crop to the upper world.
