Istros was a Mylesian colony on the northern Greek island of Thrace. The large coinage output suggests to historians and archeologists that it was a place of commercial importance. It is still a mystery as to what the obverse of this coin signifies. The most widely accepted theories are that it represents either the Dioskouri, the rising and setting sun, or the two branches of the Danube River, also called the Ister.
The Dioskuri seems to be the most popular with the public. The Dioskouri is the classical mythological name for the twin heroes Castor and Pollux. Castor and Pollux were famous warriors, and devoted brothers; Pollux renowned as a boxer, and Castor as a horseman. Pollux, granted immortality by Zeus, and rather than be parted from his brother when he died, persuaded Zeus to allow them to lead alternating lives, one day as gods on Olympus, and the next as dead mortals in Hades. In legends, Zeus created the constellation Gemini in their honor. Patrons of all seafarers, the Diskouri were especially honored by the Greeks.