No Labour of the Magnificent Herakles intrigues me more than the Mares of Diomedes, partially because I enjoy finding the truth behind the ancient stories. Diomedes was a King of Thrace, a country above Turkey (then Asia Minor) that just barely crosses the border over into Northeastern Greece. Diomedes ruled a tribe there known as the Bistones. There in a City called Tirida (which is now extinct and scattered in ruins), he kept four magnificent but also terrifying and deadly horses known as the Mares of Diomedes. They were said to breathe fire, foam at the mouth, and eat humans. Not only were they feared by man, but detested by Zeus, and they were kept chained with iron and bronze. What was the deal with these horses? What were they? Let's examine.
While human flesh is certainly not in the diet of horses, they will consume it if they're hungry enough. We also know it's possible for tortured animals to be bred to violence and barbarism. Like any other creature, when put to the right extremities, their personality and the lengths they are willing to go to can change. The foaming at the mouth is even more intriguing. Rabies first appeared around 4,000 BCE. According to Herodotus, Herakles lived around 1,300 BCE. So the disease would have had ample time to spread among animals and humans and go through mutations, especially with no vaccinations to counter it. Foaming at the mouth is a common symptom of the disease in animals, and the infection, going untreated for a long time, could have turned the horses very hostile. Rabies will cause animals to attack humans and behave out of character (such as an unusual diet perhaps).
In short, yes, the Mares of Diomedes existed. The breathing of fire may be a metaphor, but the existence of a group of horses turned ferocious, is absolutely possible. The story of the Labour comes to a tragic but also a triumphant end. After securing the Mares, Herakles left them alone with His good friend Abderus, unaware of the extent of their barbarism. When He returned, they had killed and eaten him. In revenge, Herakles threw Diomedes to his own man-eating horses. To honor His late companion, Herakles founded a City called Abdera on the grounds of the tomb where His friend rested.
There are conflicting accounts as to what happened to the beasts. Some say they were sacrificed to Zeus, but He refused to take them, and that they spent their remaining days roaming the countryside. What became of them after that is unknown. Dead now, of course. But perhaps we shouldn't be too quick to think that mares with such ferocity could never again return.
In the Goodness of the Gods,