A theory I've heard people talk about in a superhero mmorpg is that the myths of King Arthur draw inspiration from some aspects of Norse mythology.
From a total layman's perspective, there's appeal there: drawing a stone sword that none other can lift is not entirely unlike Mjolnir as of the Marvel depicition being non-lifteable by those who are not worthy.
There's a sagely wizard type in both characters' lives: while Merlin draws from Myrddin Wyllt, there are depictions of Odin as a bearded wizarddy type...
And both are warrior types, typically portrayed as bearded sorts with long hair, and then there's the symbol connection: Arthur's sometimes shown as a Christian king with crosses being prominent, while Thor's symbol is basically a T-shaped hammer.
I don't know if there's anything to this connection, but it seems not entirely unlikely to my mind there's inspirations flapping about the general mythic consciousness from older myths until Arthur's invented.
The primary basis of the Arthurian legends, in their first appearance, are the semi historical (and possibly fictional) medieval accounts of a British (not English) war chief (sometimes referred to as dux bellorum, warlord) named Arthur, who supposedly defended Britain against the invading Saxon forces c. 500 AD. It should be noted that these medieval accounts claim to be translations or collections of history in folk memory and ostensible older Welsh or British-language manuscripts. The legends sprang up around this figure and the warriors whose names were associated with him, and were heavily embroidered and added to by French authors in the centuries after the original stories began to be widely known. Some of the tales, places, and items that were earliest associated with King Arthur are extremely fantastic in the sense of fasiry tale, but some seem to be at least somewhat related to things that probably happened (some of the battles, for instance.) But the ancient British, the original native peoples of Britain, and Romanized (Christianized, even, according to most of the original sources) native Britons no less, did not look for their history or their mythology from the Norse people or Norse religions. The Arthurian legends are British in origin, not Norse, and Thor isn't Arthur nor vice versa. Sorry this is short and sourceless, I'm on mobile ATM. Will try to come back and elaborate later when I can get to a computer.
(and also their names being similar, forgot to mention that)