Mares of Diomedes

In the mythic corpus the horses are simply the herd kept by Diomedes of Thrace; multiple traditions attempt to explain their murderous appetite. Variant etiologies recorded in later summaries and commentaries include: Diomedes training the animals to eat human flesh; the horses consuming a local plant that made them carnivorous; and drinking from the river Cossinitus which produced the behaviour. Sources do not converge on a single canonical origin; early poetic evidence (a fragment attributed to Pindar) frames them as chained domestic animals kept by Diomedes until Heracles distracts and breaks their restraint, while later narratives expand details (companions killed, named horses, or genealogical links to other mythic horses).

Primary and consistent depiction is as powerful horses (commonly called mares in the literary tradition) shown in art and description rearing, pulling chariots, or carrying off human victims. Vase imagery (Late Archaic examples attributed to painters such as Psiax and Oltos) often shows one to several horses with human body parts in their mouths; two surviving objects (a fifth-century BCE lekythos by the Marathon Painter and a circa-500 BCE scarab) present rare winged-horse variants. A Roman author (Lucretius) records a singular literary variant that mentions fire-breathing from their nostrils, but this is attested only rarely and treated as a variant rather than the standard trait. The Greek formulaic name used in sources is Διομήδους ἵπποι, literally 'the horses of Diomedes.'

Their central and consistently attested ability is carnivory: they eat human beings, specifically described in sources as consuming Diomedes' unsuspecting guests. Narrative accounts emphasize their lethality and danger to humans; artistic depictions underline this by showing dismembered limbs in their mouths. Some late or isolated traditions add extraordinary attributes (occasional winged depiction in a small number of artifacts; a single literary mention of fire-breathing), but these are variant and not primary. The horses are strong and difficult to control; narratives require Heracles' strength and strategy to break their confinement and lead them away. In some versions, after capture, they continue to be dangerous—killing Heracles' companion Abderus in fuller accounts.