Tolkien being an anglo-saxon scholar published his translation of Sir Gawain and the Green Knight, which is still a standard to medieval literature. Gawain also happens to be an earlier story in the Arthurian legends. I hate to burst anyone's bubble, but King Arthur did not really exist. He was created to unite the clan structures under one rule- a king- something that the extended clans could not fathom. Giving up allegiance to family for someone they didn't even know or a large land mass they did not own or work? So Arthur was born and Merlin was the druidic figure to bridge the pagan ways to the Christian ones that were being imported from the south (Italy, Spain, etc.)

Spenser's Faerie Queene, Orlando Furiouso (the first 'Gothic novel'), "Sir Orfeo", the Pearl, Piers Plowman, Tristan & Isuelt, Troilus & Cressida, Everyman, the "Wanderer, " the "Seafarer," etc. etc. are literary examples of the romantic age when dragons, mystical beings, and a serious code of ethics to live by existed.

A piece of literary piece of trivia for you; elves and other "fairy" type magical beings were believed to be human sized until William Shakespeare wrote "A Midsummer Night's Dream" and made Hippolyta's handmaiden the size of a mustard
seed.
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Here's my personal list of works that influenced Tolkien and his mythology:

'Plato, The Republic' - The story of Gyges the Lydian is part of book II of the Republic. A ring is born!!!!

'The Kalevala' - Elias Lonnrot - This Finnish epic inspired the language of the Middle Earth elves.

'Beowulf: A New Verse Translation' - Seamus Heaney (translator)- Anglo-saxon warrior history and myth blended exquistely.

'Sir Gawain and the Green Knight, Pearl, Sir Orfeo' - Theme of temptation. Tolkien translated a version and was very familiar with this 14th century classic. (lots of translations out there so check around).

'The New Oxford Annotated Bible: With the Apocrypha/Deuterocanonical Books/New Revised Standard Version '- Duh...good and evil....

'J.R.R. Tolkien and His Literary Resonances: Views of Middle-earth (Contributions to the Study of Science Fiction and Fantasy)'

'Proceedings of the J.R.R. Tolkien Centenary Conference, 1992: Proceedings of the Conference Held at Keble College, Oxford, England, 17Th-24th August 1992 to Celebrate the Cenenary of the Birth of Conference (myth (Mythlore Series)'

'The Red Fairy Book' - Andrew Lang - A seven-year-old Tolkien is inspired by Sigurd killing Fafnir, the dragon.

'Macbeth' - W. Shakespeare - The Ents mirror Birnam Wood coming to Dunsinane

'Roverandom' - Wayne G. Hammond

'The Iliad: The Story of Achilles' - Homer, W. H. D. Rouse (Translator)- the staves of the wizards