Amphion and Zethus
A secondary pair of twins fathered by Zeus, Amphion and Zethus, the legendary founders of Thebes, are called "Dioskouroi, riders of white horses" (λευκόπωλος) by Euripedes in his play "The Phoenician Women" (the same epithet is used in "Heracles" and in the lost play "Antiope").