The British warrior whose prowess is compared to Arthur’s in Y Gododdin:
he glutted black ravens [slew his enemies] on the ramparts of the stronghold, though he was not Arthur.
This passage refers to a battle fought at Catraeth (possibly Catterick) in about AD 600. In the battle, the British were destroyed by the Angles. The passage in question signifies that though Gwawrddur was mighty, he was not as mighty as Arthur. This is the earliest existing appearance of Arthur’s name.
Clearly, the audience of the poem was expected to know the identity of “Arthur,” conventional scholarship holds that the reference alludes to the famed battle-leader of a century prior, but some scholars (e.g., Richard Barber, Figure, 21-34) have argued that the passage refers to Arthur of Dalriada, a northern figure who lived contemporary to the writing of Y Gododdin, and in close geographic proximity to its writer and audience. Some have suggested, too, that the passage is a later interpolation in the poem, subsequent to Arthur’s fame.
Source
Y Gododdin | Aneirin, c. 600