The creature's name appears in variant forms across the ancient sources. Hesiod, the oldest literary source, uses the spelling "Orthus" (Ὄρθος, Orthos), while the mythographer Apollodorus uses "Orthrus" (Ὄρθρος, Orthros). Both spellings appear throughout ancient texts and refer to the same two-headed hound of Geryon.

According to Hesiod, Orthrus fathered the Sphinx and the Nemean Lion, though the identity of their mother — whether Echidna, the Chimera, or Ceto — remains unclear in the text.

Together with the herdsman Eurytion, Orthrus guarded the red cattle of the three-bodied giant Geryon on Erytheia, an island in the far west of the Mediterranean. As his tenth labour, Heracles killed Orthrus — with his club according to Apollodorus, or by arrows in artistic depictions — then slew Eurytion and Geryon and drove off the cattle.

The poet Pindar uniquely refers to the "hounds" of Geryon in the plural, possibly because Orthrus possessed multiple heads or reflecting an alternate tradition.

Depictions of Orthrus in visual media are uncommon and consistently accompany the narrative of Heracles' theft of the cattle. The creature typically appears dead or dying, often pierced by arrows.

The earliest artistic representation appears on a late seventh-century BC bronze horse pectoral from Samos, depicting a two-headed Orthrus with an arrow in one head, crouching before Geryon while confronting Heracles. A red-figure cup by Euphronios (Munich 2620) shows Orthrus lying belly-up with an arrow through his chest and a writhing snake tail, positioned before the three-bodied Geryon and the lion-skin-clad Heracles. An Attic black-figure neck amphora by the Swing Painter depicts the two-headed Orthrus at Geryon's feet with two arrows piercing one head and a dog tail visible.

While Apollodorus specifies two heads, artistic representations vary. Orthrus is most commonly shown two-headed, as on the Samos pectoral, Euphronios' cup, and the Swing Painter amphora, but from the mid-sixth century onward some images show single-headed versions, and one early fifth-century Cypriot stone relief depicts three heads, resembling Cerberus. The Euphronios cup and the stone relief portray Orthrus with a serpent tail like Cerberus, though most depictions give him a canine tail.

Orthrus bears a striking resemblance to Cerberus, the hound of Hades, and the classical scholar Arthur Bernard Cook termed Orthrus the "doublet" of Cerberus. According to Hesiod, both descended from Echidna and Typhon, and both possessed multiple heads — Cerberus had fifty or one hundred heads in early accounts, later standardized to three in literature, though artistic renderings often show two.

Both monsters typically appeared with serpent tails and functioned as guard dogs — Cerberus at the gates of Hades, Orthrus over the cattle of Geryon — and each was overcome by Heracles in the course of his labours.