Some people believe that King Arthur is so inextricably tied up in Celtic Mythology that he must, in origin, have been, not a man at all, but a God. Like so many other characters featured in the Mabinogion, Arthur in his earliest form, appears almost entirely mythical. He and his companions have superhuman strength and abilities, and consort with giants and other mythological creatures. In the early Welsh poem "Preiddeu Annwfn", Arthur visits the Celtic Underworld, Annwfn, and his adventures closely parallel those of the cauldron-seeking God, Bran the Blessed. Even in Geoffrey and Malory, upon being fatally wounded in battle, Arthur is carried to the mystical Avalon, apparently the Underworld home of the Celtic god, Afallach. Many legends around the country attest to Arthur's immortality. He sleeps in one of numerous caves waiting to return and lead his people.
The name Arthur itself appears to derive from the Celtic word Art, meaning "bear". Could Arthur, like so many other Celtic gods, be merely a personification of the many reverred animals of the wild? Later to become humanized like Loucetios, one of several Celtic deities known to be able to transform themselves into birds or beasts of the forest. Many such gods had stellar associations and the constellation of Ursa Major or the Great Bear is sometimes known as Arthur's Wain even today.
Three Bear-Gods are known from the Celtic World. Strangely, they acted as both champion of bear-hunters and protectors of the beast itself. The most celebrated was, perhaps, Artio, worshipped near Berne in Switzerland and around Trier; but she was actually a goddess. A male god, Artaios, was reverred in Beaucroissant in Isere, where he was identified with the Roman Mercury. In Britain there is scant evidence for the bear cult, though a number of small jet bear talismans from Yorkshire may have devotional associations. The god to which they probably relate, however, derives his name from the alternative bear word, matus (Gaulish) or math (Irish). Matunus appears to have had a shrine at Risingham, just north of Hadrian's Wall.
Some theorists claim Arthur was a late addition to the Celtic pantheon
during a resurgence in pagan worship, or possibly a mythical hero,
the offspring of a human and a bear.