In Celtic mythology who possessed a magical steed named Aonbarr?

This is the name of a fabulous horse in the mythology and folklore of Ireland. Aorbárr, whose name in Irish Gaelic means " Unique Supremacy," is also known by the name Enbhárr, which may be translated a "Froth" or "Foam." He is the steed of Manannán mac Lir and, as the name implies, was capable of dashing over any surface, whether mountain, marsh, or sea. ---- Mackillop, J. Dictionary of Celtic Mythology. Oxford: Oxford University Press, 1998.