How Do We Know That?

The earliest known visual representations of king arthur are crucial to this question. The modena architrave shows an episode which does not appear at all in geoffrey of monmouth's book. And the otronto mosaic we see there arthur doing battle with cathpalug, the giant cat that eventually killed and replaced him. So we can see there was the circulation of some independent body of lore and narratives about arthur. Most likely spread by bretons, maybe by bards and minstrels or others,. Maybe knights servants who travelled through france and italy during the crusades.