Castor and Pollux

In Greek and Roman mythology, Castor and Pollux, or Kastor and Polydeuces, were twin brothers, together known as the Dioscuri or Dioskouroi. Their mother was Leda, but they had different fathers; Castor was the mortal son of Tyndareus, the king of Sparta, while Pollux was the divine son of Zeus, who seduced Leda in the guise of a swan. Though accounts of their birth are varied, they are sometimes said to have been born from an egg, along with their twin sisters or half-sisters Helen of Troy and Clytemnestra.

In Latin the twins are also known as the Gemini or Castores. When Castor was killed, Pollux asked Zeus to let him share his own immortality with his twin to keep them together, and they were transformed into the constellation Gemini. The pair were regarded as the patrons of sailors, to whom they appeared as St. Elmo's fire, and were also associated with horsemanship.

They are sometimes called the Tyndaridae or Tyndarids, later seen as a reference to their father and stepfather Tyndareus.

The best-known story of the twins' birth is that Zeus disguised himself as a swan and seduced Leda. Thus Leda's children are frequently said to have hatched from two eggs that she then produced. The Dioscuri can be recognized in vase-paintings by the skull-cap they wear, the pilos, which was explained in antiquity as the remnants of the egg. Whether the children are thus mortal or half-immortal is not consistent among accounts, nor is whether the twins hatched together from one egg. In some accounts, only Pollux was fathered by Zeus, while Leda and her husband Tyndareus conceived Castor. This explains why they were granted an alternate immortality. It is a common belief that one would live among the gods, while the other was among the dead. The figure of Tyndareus may have entered their tradition to explain their archaic name Tindaridai in Spartan inscriptions or in literature Tyndaridai, in turn occasioning incompatible accounts of their parentage.

...
Wikipedia