Cerberus

Cerberus guards the entrance to the house of Hades. The number of heads varies by source — Hesiod gives him fifty, later writers settle on three — and some describe a mane of serpents and a serpent's tail. He is a child of Typhon and Echidna, which makes him brother to the Hydra, the Chimera and the Sphinx: the same litter produced most of the monsters in Greek myth.

His job is precisely defined and frequently misunderstood. He is not there to keep the living out; he is there to keep the dead in. The old descriptions have him fawning on new arrivals like any dog greeting company, and turning on anyone who tries to leave.

He was got past four times, and never twice the same way. Heracles took him by main force as the twelfth and final labour, wrestling him up into daylight without weapons and then returning him, which is the only labour that ends with the prize handed back. Orpheus played him to sleep. The Sibyl who guided Aeneas drugged him with a honeyed cake — the origin of the phrase 'a sop to Cerberus', a bribe paid to get past a guard. Psyche did the same on Aphrodite's errand.

The pattern is the point. Force, music, and a bribe all work on him, and each says something different about the hero attempting it.