Castor and Pollux, the Dioscuri, were the twin sons of Leda, one mortal and one immortal. Castor's father was Tyndareus, the King of Sparta. Pollux' father was Zeus, in the form of a swan.
Pollux asked his father, Zeus, to grant Castor half his immortality when Castor was killed. Castor and Pollux then transformed into the constellation Gemini.
Castor and Pollux are consistently associated with horses in art and literature. And here on coinage. They were regarded as patrons of athletes and sailors.
THE HIPPOI DIOSKOUROI (Horses of the Dioscuri) were four immortal steeds gifted to the twins by the gods Hera and Hermes. They served the twins both during their time on earth and after their ascension to godhood. The horses were named Kyllaros, Xanthos, Phlogeus and Harpagos. (ref: theoi.com)
