The famous four children that resulted from Leda mating with Zeus in swan form

For those who've forgotten their ancient mythology: after Leda made it with the swan—really Zeus/Jupiter in disguise—she laid two egg out of which hatched four kids—two the progeny of Zeus, the other two the progeny of Leda's husband Tyndareus, the king of Sparta. (It's myth; just go with it.)

Two of the brood were identical brothers known as the Dioscuri, Castor (mortal) and Pollux (demigod—in Greek, his names was Polydeuces). The twins would go on to have many high-profile adventures—including joining Jason's crew of Argonauts, and participating in the hunt of the Calydonian Boar—and are known as the protectors of travelers. Upon their death, Zeus offered his son Pollux the interesting choice to split his immortality with his mortal brother, so they now alternate their time between Hades and Olympus—and serve as the brightest two stars in the Gemini constellation.

The other two egg-born children were daughters, also split between mortal and demigod: Clymensetra—the mortal who would go on to marry King Agamemnon—and a half-divine woman named Helen, who would grow up to be a real beauty, get kidnapped, and cause the whole Trojan War debacle ("the face that launched a thousand ships"). 

At the end of that war, Aeneas, fleeing a burning Troy, would have a series of adventures (chronicled in Virgil's masterful propaganda poem The Aeneid) and eventually go on to build the city of Rome

The Romans, counting back a few steps, decided they really dug the whole Leda story and often featured it in art.