Penath, Vale of Glamorgan, Wales
Penarth is in the hundred of Dinas Powys. I had wondered about this containing an actual arth or "bear" name, rather than being composed of Pen + ardd for "End of the Height", or some other suitable (and more prosaic) description for a headland. When I researched the place-name, I found this by local history expert Phil Carradice:
Pen means “head of” so the seaside town of Penarth means “Head of the Bear.” Ships captains beating up the Channel used to say that the headland and landmark on the coast before Cardiff was shaped like a bear. Consequently, when the town and its docks were formed in the mid nineteenth century Penarth was a logical name to call it.
When I contacted Mr. Carradice, he was kind enough to refer me to his source for this information:
"Roy Thorne (an uncle, I think, of Alan Thorne) published a book on the history of Penarth back in 1975 and I used that as the basis for the article."
If the bear etymology is correct, then Arthur as a decknamen may well have replaced an earlier Irish Artri or British Arthr(h)i, and the latter may well have been a reference to the 'Bear's Head' near the Dinas Powys hillfort of Illtud the Terrible Chief-dragon.