As Owain, he appears in the Welsh tale of The Dream of Rhonabwy and is the hero of Chrétien de Troye's Yvain or the Knight of the Lion.
Both tell of Yvain (or Owain) adventures. Yvain defeats a knight guarding a fountain and marries the dead knight's wife. Gawain persuades him to then journey abroad in seach of adventure. One of his adventures involved saving a lion from a serpent. The grateful lion befriends Yvain and helps him against those trying to destroy him.
The English romance Yvain and Gawain is adapted from Chrétien's Yvain. Yvain features in the French Mort Artu; and in Malory's Morte D'Arthur when Arthur banishes Yvain from court because of Morgan le Fay's trying to kill Arthur. Gawain and Marholt leave with him, and they have a series of adventures with three ladies that they then meet.
Full references to more about Yvain at the Rochester site
Characters from King Arthur's legends