According to Horace, Cerberus possessed one hundred heads. Hesiod wrote that he had fifty, while most sources agree to only three. The center head was in the shape of a lion, while the other two were in the shape of a dog and a wolf, respectively. He also had a dragon's tail and a thick mane of writhing snakes.
It is generally thought that Cerberus was born to Echidne, a half-woman, half-serpent, and Typhon, the most fierce of all creatures.
Cerberus has a brother, Orphus, which is also a monstrous dog with two heads. Cerberus’ Egyptian correspondent is Anubis, the dog who guarded the tombs and conducted the souls to the underworld.
A similar dog, Garm, is guarding the house of deaths in the Norse mythology. These monsters were probably inspired from the dogs that haunted the battlefields in the dark of the night, feasting on the bodies of the fallen warriors.
The three heads relate to the threefold symbol of the baser forces of life. They represent the past, the present and the time yet to come. Dante described Cerberus as “il gran vermo inferno” thus linking the monsters with the legendary worms and orms.
Cerberus is the watchdog of Hell. He is often pictured with Hades, his master. He can be found on the banks of the river Styx, where he had the task of eating any mortals who attempted to enter, and any spirits who attempted to escape.
As Cerberus vehemently resisted Heracles, barking furiously, his saliva dripped on the ground, giving birth to a poisonous plant called aconite; thus named because it flourishes on bare rocks. It is also known as 'hecateis,' because Hecate was the first to use it. Medea tried to poison Theseus with it, and the Thessalian witches used it in preparing the ointment that enabled them to fly. The modern name for aconite is wolfsbane.
Ancient Greeks and Romans placed a coin and a small cake in the hands of their deceased. The coin was meant as payment for Charon who ferried the souls across the river Styx, while the cake helped to pacify Cerberus. This custom gave rise to the expression 'to give a sop to Cerberus,' meaning to give a bribe or to quiet a troublesome customer.