In Sir Thomas Malory’s Le Morte d’Arthur, a king named Ryons sends word to Arthur that he has collected the beards of eleven subservient kings and put them on his cloak. He tells Arthur to shave his own and send it in tribute to him. Arthur of course refuses and Ryons is subsequently defeated, eventually becoming a part of Arthur’s court.
Names in Arthuriana are notoriously difficult to trace, but it seems likely that Ryons first appears as Rhitta in a Welsh tale. The story begins with two brothers arguing over who has the better kingdom. Hearing of their folly, Rhitta conquers their kingdoms and forces them both to shave their beards as homage. With these he makes a cap. Twenty-six nearby kings hear of his unusual punishment and attack him out of fear. They fail, as does a third and larger host. With all of these he makes a cloak. Rhitta then decides that Arthur should shave his beard as well, before he begins to act as the brothers had. This does not end well for Rhitta, who ends up adding his own beard to the cloak.
Geoffrey of Monmouth is the next writer to mention the giant Ritho/Rhitta, saying little more than that he demanded Arthur’s beard and was defeated. Moving forward he was Wace’s giant Rithon, Chrétien de Troyes’ Rion of the Isles, and the Ryons to be found in the Prose Merlin and Vulgate. Each of them first appear as Arthur’s competition or are implied as such and several of them are assigned the same beard and cloak motif.
In Celtic legends, enemy kings are often remembered as giants to be defeated by heroes. It is also well established that many figures who were significant legendary or historical figures in their own right were drawn into Arthur’s orbit. This process began with those closest to him chronologically and geographically and expanded to include a large portion of Britain from Welsh myth through the twelfth century
Urien was one of the more powerful kings of the post-Roman era; he lived within a hundred years of Arthur. Urien’s power was centered in Reged, modern Cumberland, or in the same region as Arthur. In Welsh, the letter u sounds like a long e, making it easy to omit. Thus we have Rien, pronounced Rēe-en. With as unpredictable as many personal names can be in Arthuriana, it is easy to see how Urien could have been transformed into Ryon in the oral of transference to the continent. Ryon is closely associated with Welsh Rhitta, but the comparison by way of beard motif seems fairly certain.
As to an historical context for the beard motif, there doesn’t appear to be one. Urien is the subject of several legitimate Taliesin poems, is mentioned as the leader of a British coalition in Historia Brittonum, and is found in the Welsh Triads, but nowhere is there any hint of him asking for beards as homage. This does not mean he did not originate that connection, but it is just as likely that the idea came from a bard at some time after Urien and before the writing of Geoffrey of Monmouth’s Historia Regum Britanniae.