Was Arthur a Bear?

I'm rereading Stewart's THE CRYSTAL CAVE for the first time in decades, and enjoying it a lot. (It's the book that infected me with the Arthurian virus long ago. I caught a bug from you, book, and I don't need no cure, etc.)

But I was struck by a section where Stewart (or one of her characters) suggests that _Arthur_ means "bear" (relating it to Greek ἄρκτος).

It's an appealing folk-etymology, and maybe just that, but I wonder if there could be something to it. This would tie Arthur to a widespread tradition of late-antique/early-medieval bear heroes (e.g. Beowulf, Bodvar Bjarki, etc).

But I know nothing about the etymology of the Wart's name, and very little about the development of the Celtic branch of Indo-European, so it might be just my ignorance talking here.

The tyrant OED was no use (as is usually the case when it comes to slicing up pieces of *PIE), but the AHD had this to say.

“r̥tko-
Bear. Oldest form *h2r̥tk̑o-, becoming *h2 r̥tko- in centum languages.
1 ursine, from Latin ursus, bear (< *orcsos).
2 arctic, Arcturus, from Greek arktos, bear.
[Pokorny ŕ̥k̑Þo-s 875.“

I’m not asking anyone to do my homework for me, but any hints or suggestions would be gratefully accepted.

J(“JE suis un monstre”)MP