Apart fom Nennius' extremely suspect account, no narrative of Arthur whatsoever exists until Geoffrey of Monmouth (1135) wrote the Arthurian Stories. Apart fom the fact that much of the material in Geoffrey's tales were apparently compiled from a variety of sources (from fairly widely spaced traditions) the stories appear to have been basically invented by Geoffrey of Monmouth himself.

Previous "sources" are as follows.

The battle of Badon (c500ad) was mentioned by the Welsh writer Gildas relatively soon after it happened (mid 6th Century) but Gildas did not mention anyone called Arthur.

Some 300 years later, Nennius' Historia Brittonum (completed circa 800ad) mentions a "hero-warlord" called Arthur, associated with the battle at Badon. In translation ... Arthur fought against them in those days with the kings of the Britons, but he himself was leader of battles. The Arthur of Nennius' account, however, shows clear signs of being "lifted" from some unknown poems and it's clear that he placed his "Arthur" into a historical context which does not fit him.

The Annales Cambriae (Annals of Wales), circa 970 ad, has a single mention of Arthur, based on Nennius' account.

William of Malmesbury, 1125, embroiders on Nennius' account of Arthur as does Henry of Huntingdon (approximately the same date).

In conclusion, no Arthurian story is pre 12th Century and all of them are based on the chivalric ideas and beliefs of this period.

No sense in going looking for anything so early as Beowulf. Neither is there any sense in looking for an Ancient Briton as that's not who the hero of our story was. Best to look at the Norman aristocracy on which the Arthurian stories are lagely modelled.

:-)