Castor and Pollux, in Roman mythology, were the twin demi-god offspring of Jupiter and Leda and equivalent to the Dioscuri of Greek mythology. They are also represented in the constellation Gemini. Pollux was considered immortal whilst his brother was mortal; both were particularly associated with cavalry and the cult of the twins went back to the mid-6th century BCE in Italy. The original temple in Rome was dedicated by the Roman general Aulus Postumius who won a great victory against the Latins in the Battle of Lake Regillus (499 or 496 BCE).
During the battle two young men riding white horses were said to have appeared and guided the Romans to victory and then were seen again after the battle watering their horses at the Juturna Spring in Rome, hence the subsequent dedication to the famous cavalry twins and choice of location for the temple next to the fountain. Parts of the original temple's podium survive and hint at its great size. Every 15th of July the temple was the focus of a cavalry parade - the transvectio - of 5,000 men led by two impersonators of the heroes who commemorated the victory at Regillus.