Castor and Pollux (or Polydeuces) were the twin sons of Leda and Zeus and the brothers of Helen of Troy. Because Zeus came to Leda in the form of a swan, they are sometimes presented as having been born from an egg. Pollux was a formidable boxer, and Castor was a great horseman. Together, they were the "Heavenly Twins," often associated with the constellation Gemini.
Pollux persuaded Zeus to share his gift with Castor. As such, the two spend alternate days as gods on Olympus and as deceased mortals in Hades.