Cerberus

Cerberus (Ancient Greek:Κέρβερος), or Kerberos, in Greek mythology and Roman mythology, is a three-headed) dog, or "hellhound" with a serpent's tail, a mane of snakes, and a lion's claws. He guards the entrance of the Underworld to prevent the dead from escaping and the living from entering. Cerberus is featured in many works of ancient Greek and Roman literature and in works of both ancient and modern art and architecture, although the depiction of Cerberus differs across various renditions. The most notable difference is the number of his heads: Most sources describe or depict three heads; others show Cerberus with two or even just one; a smaller number of sources show a variable number, sometimes as many as fifty or even a hundred.

In myth
Cerberus is the offspring of Echidna, a hybrid half-woman and half-serpent, and Typhon, a gigantic monster even the Greek gods feared. Its siblings are the Hydra; Orthrus, a two-headed hellhound; and the Chimera, a three-headed monster. The common depiction of Cerberus in Greek mythology and art is as having three heads. In most works, the three heads respectively see and represent the past, the present, and the future, while other sources suggest the heads represent birth, youth, and old age.

The Twelfth Labour of Heracles
Capturing Cerberus was the assigned to Hercules by King Eurystheus, in recompense for the killing of his own children after he was driven insane by Hera, and therefore was the most dangerous and difficult.

After having been given the task, Heracles went to Eleusis to be initiated in the Eleusinian Mysteries so he could learn how to enter and exit the underworld alive, and in passing absolve himself for killing centaurs. He found the entrance to the underworld at Tanaerum, and Athena and Hermes helped him to traverse the entrance in each direction. He passed Charon with Hestia's assistance and his own heavy and fierce frowning.

Whilst in the underworld, Heracles met Theseus and Pirithous. The two companions had been imprisoned by Hades for attempting to kidnap Persephone. One tradition tells of snakes coiling around their legs then turning into stone; another tells that Hades feigned hospitality and prepared a feast inviting them to sit. They unknowingly sat in chairs of forgetfulness and were permanently ensnared. When Heracles had pulled Theseus first from his chair, some of his thigh stuck to it (this explains the supposedly lean thighs of Athenians), but the earth shook at the attempt to liberate Pirithous, whose desire to have the wife of a god for himself was so insulting, he was doomed to stay behind.

Heracles found Hades and asked permission to bring Cerberus to the surface, to which Hades agreed if Heracles could overpower the beast without using weapons. Heracles was able to overpower Cerberus and proceeded to sling the beast over his back, dragging it out of the underworld through a cavern entrance in the Peloponnese and bringing it to Eurystheus.