Mares of Diomedes
The Mares of Diomedes were four man-eating horses in Greek mythology. Magnificent, wild, and uncontrollable, they belonged to the giant Diomedes, king of Thrace, a son of Ares and Cyrene who lived on the shores of the Black Sea. Bucephalus, Alexander the Great's horse, was said to be descended from these mares.
When the sun is in the constellation of Aquarius, the constellation Pegasus rises. Pegasus in early Greece was considered to contain 4 very bright stars, making a square, it was only in later times that the 4th star (Alpheratz) was considered part of Andromeda. By reassigning the 4th star, Pegasus changed from being a horse with a square body, into being a horse with a wing (the square body changing into a triangular wing), giving rise to the winged horse myth.
Bright stars were considered to be malevolent and wild, thus leading to the earlier pegasus square being considered 4 evil horses (the animals being horses due to the overall shape assigned to the constellation). Pegasus, as a whole, appears to be feeding, in particular, it aims its head towards Aquarius, a man, suggesting a man-eating nature. Since the horses are above the ecliptic, they cannot be said to have died, and thus must have been caught, since the sun is able to pass them.=
Aquarius itself was said to represent the god who flooded the earth; the water it seems to pour, which sometimes includes the constellation of Eridanus as a river, was said to depict this by the Greeks. Some versions of the myth of the Mares of Diomedes hold that Herakles created a river around the stable of the mares.
According to the Trojan War epics, King Diomedes, the hero of Argo, who fought against the god Ares during the war, stole horses from the stables of King Rhesus of Thrace. Since Herakles was the hero of Argo, and since Diomedes (of Argo) was perceived as already owning the horses, Diomedes had to take the place of Rhesus in the story, becoming the villainous enemy of Argo, descended from Ares.
The Eighth Labour of Heracles
One of the Twelve Labours of Heracles was to steal the Mares. In one version of the story, Heracles brought Abderus, one of his many male lovers (eromenos), and some other youths to help him. They took the mares and were chased by Diomedes and his men.
Heracles was not aware that the horses were kept tethered to a bronze manger because they were wild, man-eating and uncontrollable, and Heracles left Abderus in charge of them while he fought Diomedes, but Abderus was eaten. In revenge, Heracles fed Diomedes to his own horses, then founded Abdera next to the boy's tomb.
In another version, Heracles stayed awake so that he didn't have his throat cut by Diomedes in the night, and cut the chains binding the horses. Having scared the horses onto the high ground of a peninsula, Heracles quickly dug a trench through the peninsula, filling it with water, thus making it an island. When Diomedes arrived, Heracles killed him with an axe (the one used to dig the trench), and fed the body to the horses.
Eating made the horses calmer and Heracles took the opportunity to bind their mouths shut, and easily took them back to King Eurystheus, who dedicated the horses to Hera. In some versions, they were allowed to roam freely around Argos, having become permanently calm, but in others, Eurystheus ordered the horses taken to Olympus to be sacrificed to Zeus, but Zeus refused them, and sent wolves, lions, and bears to kill them.