Two entries with the name Gwendolen.
According to Sir Walter Scott’s The Bridal of Triermain (1813), Gwendolen was the half-fairy mother of Gyneth by Arthur, and it seems reasonable to assume that Scott simply picked the name from the earlier legends.
When Arthur bid his fairy lover adieu she gave him a farewell-cup from which a drop happened to fell and landed on the king’s horse who got so badly burned it went crazy and died. The king threw the cup to the ground and the grass started to burn and Gwendolen’s palace burnt down.
The Bridal of Triermain | Sir Walter Scott, 1804
A lover of Merlin whom he turned into a hag after she rebuked him for his demonic powers.
Gawain later married her and, when he kissed her, she returned to her beautiful form.
Dame Ragnell | The Legend of King Arthur
The Masque of Gwendolen | Reginald Heber, 1816