The Onomasticon Anglo-Saxonicum lists an
Artur,
Arthur, Arnthur and Arthor, which the editor assumes are variants of Earthur/Earnthur (Ear/Earn "eagle/erne" + Thur "
Thor"). This seems to be a peculiarly British combination. This makes the Arthurian romances pure fantasy (only to be expected, they are romances after all), but for some reason people want some historical basis which requires
Arthur to be British, not
Saxon (The irony being we now recognize that the "Anglo-Saxons" were largely British themselves).