The original Knight of the Swan was Lohengrin.
LOHENGRIN (lō’en-grin), the hero of an old High German poem, written in the end of the 13th century. He was the son of Parzival, and a knight of the Grail. At King Arthur’s command he was taken by a swan through the air to Mainz, where he fought for Elsa, daughter of the Duke of Brabant, overthrew her persecutor, and married the lady. Then he accompanied the emperor to fight against the Hungarians, and subsequently warred against the Saracens. On his return home to Cologne, Elsa, contrary to his prohibition, persisted in asking him about his origin. After being asked a third time he told her, but was at the same time carried away by the swan back to the Grail. Rückert’s edition (1857) of the poem is the best. The poem is a continuation of Wolfram von Eschenbach’s Parzival. Wagner made it the subject of his great opera, “Lohengrin” (1848).
Collier’s New Encyclopedia, 1921.
There are many medieval versions of the Swan Knight story. Here’s one I particularly like:
“In the year 711 lived Beatrice only daughter of Dietrich, Duke of Cleves, at her castle of Nymwegen. One bright day she sat at her window looking down the Rhine, when she saw a swan drawing a boat by a gold chain. In this vessel was Helias. He came ashore, won her heart, became Duke of Cleves, and lived happily with her for many years. One thing alone interfered with her happiness: she knew not whence her husband came, and he had strictly forbidden her to ask. But once she broke his command, and asked him whence he had come to her. Then he gave his children his sword, his horn, and his ring, bidding them never separate or lose these legacies, and entering the boat which returned for him, he vanished for ever.”