Early sources vary dramatically on the number of Cerberus's heads. Hesiod's Theogony gives him fifty heads, and Pindar reportedly gave him a hundred, but later writers almost universally settled on three heads as the standard. Horace uniquely described a single dog head crowned with a hundred snake heads, while Apollodorus reconciled the traditions by giving him three dog heads together with the heads of all sorts of snakes along his back. The Byzantine poet John Tzetzes described fifty heads, three of them canine and the rest belonging to other beasts.

A serpent element pervades the descriptions: Hesiod's mother of the monsters, Echidna, was half-snake, and writers and artists frequently gave Cerberus snakes rising from his body, a snake mane, venomous mouths, and a tail ending in a serpent's head. Additional traits attributed to him included a taste for raw flesh (Hesiod), fire-flashing eyes (Euphorion), a three-tongued mouth (Horace), and acute hearing (Seneca).

Cerberus's mythology centres almost entirely on his capture by Heracles. Eurystheus, king of Tiryns, commanded Heracles to bring the hound up from Hades, a task Apollodorus identifies as the twelfth and final labour. A fragment of the lost play Pirithous suggests Eurystheus set the task believing it impossible, not from any wish to see the beast.

While in the underworld, Heracles found the heroes Theseus and Pirithous imprisoned by Hades for attempting to abduct Persephone; in Apollodorus they were bound to the Chair of Forgetfulness and held fast by coils of serpents. Heracles freed Theseus, but when he tried to raise Pirithous the earth quaked and he let go. Accounts of the two heroes' fate vary widely.

Versions of the capture itself differ. Apollodorus says Hades allowed it only if Heracles overcame Cerberus without his weapons, so the hero squeezed the dog around the head until it submitted, using his lion-skin as a shield. Homer implies a fight in which Hades himself was wounded by an arrow; early sources describe Heracles using stones rather than iron, while later iconography shows his wooden club, and Ovid has him drag Cerberus up with chains of adamant.

Several places claimed to be the spot where Cerberus was brought up into the light — Tainaron (Strabo), Troezen (Apollodorus), and Heraclea Pontica. At the last, tradition held that Cerberus's vomit, or the poison-foam he spewed when dazzled by the unaccustomed daylight, gave rise to the poisonous aconite plant. After the capture, Heracles is sometimes said to have paraded the hound through Greece before showing him to Eurystheus as commanded and then returning him to the underworld.

The capture of Cerberus was hugely popular in ancient Greek and Roman art. One of the earliest depictions, a Corinthian cup from Argos (c. 590–580 BC), shows Heracles with quiver and bow, accompanied by Hermes, threatening Hades with a stone, while Cerberus has a single canine head and snakes rising from his head and body. A mid-sixth-century BC Laconian cup introduced the elements that became canonical: three heads, a snake tail, Cerberus's chain, and Heracles's club.

Most surviving depictions of Heracles and Cerberus occur on Attic vases. Early Corinthian examples favour a single head, Attic vase painting usually shows two dog heads, and other artwork typically shows three; Roman art sometimes gave him a large central lion head flanked by two smaller dog heads. He frequently appears part-serpent, with snakes rising from his snout, head, neck, back, ankles, and paws, and occasionally with leonine features such as a mane or claws. The subject became rare after Attic vase painting declined in the fifth century BC and was revived in the Roman period.

From the sixth century BC onward, ancient writers tried to explain the myth in naturalistic terms. The earliest such rationalization, from Hecataeus of Miletus, held that Cerberus was not a dog at all but a large venomous snake living at Tainaron, whose bite killed instantly — hence the name hound of Hades.

Palaephatus made Cerberus an ordinary dog, one of two that guarded Geryon's cattle (the other being Orthrus); because Geryon lived in a city called Tricranium (Three-Heads), both dog and master came to be called three-headed. Philochorus turned Hades into a mortal Molossian king, Aidoneus, who kept a great dog named Cerberus to which the suitors of his daughter were thrown. Heraclitus the paradoxographer claimed that Cerberus had two pups that never left his side, making him appear three-headed.

Later commentators read Cerberus symbolically. Servius, deriving the name from creoboros (flesh-devouring), saw him as the corpse-consuming earth, with Heracles's triumph representing victory over earthly desires. Fulgentius interpreted the three heads as the three origins of human strife — nature, cause, and accident — and as the three ages of infancy, youth, and old age at which death enters the world, while Eusebius made them the sun's three positions of rising, midday, and setting. The Vatican Mythographers expanded these readings, the Third Mythographer making Cerberus a figure for the three kinds of human hatred.

Virgil's ravenous Cerberus became proverbial as a symbol of avarice. Dante placed him in the Third Circle of Hell, guarding the gluttons, where he rends, flays, and quarters the spirits, until Virgil silences him by flinging handfuls of earth into his rapacious gullets.