The name Cawrnur occurs in the Kadeir Teyrnon, The Chair of the Prince, which speaks of pale horses under saddle being led from him. In the Marwnat vthyr pen, Uthr Pen[dragon]’s Elegy, there is a reference to an attack on the sons of someone named Cawrnur. Sims-Williams wrote:
“Presumably the fact that Cawrnur and Arthur rhyme partly explains their collocation, but both poems may allude to some lost Arthurian story.”
If we speculate that for the sake of rhyming Cawrnur is a variant of the individuals actual name than a reasonable candidate would be Cawrdaf ap Caradog Freichfras who was of gen 0, see St. Collen. According to triad 13 he was one of the Chief Officers of the Island of Britain. He appears as one of Arthur’s counselors in Breuddwyd Rhonabwy when Osla Gyllellfawr asked for a limited truce.
|Gen.||ByS 51||ByS J 51||ByS Y(S) 88||ByS Y(S) 89|
|2||St. Dyfnog||St. Dyfnog|
|1||Medrod||Medrod||Gwenhwyach||Iddew Corn Brydain||St. Cathen|
|-1||Caradog Freichfras||Caradog Freichfras||Caradog Freichfras||Caradog Freichfras|
|-2||Llŷr Marini||Llŷr Marini|
Gwenhwyach was the wife of Medrod. TYP 53 indicates a dispute between her and Gwenhwyfar led to Camlan. Iddog Cordd Prydain, the Embroiler of Britain, appears in Rhonabwy‘s Dream as one of the messengers between Arthur and Medrod. However, he twisted Arthur’s words when reporting them as he was keen for the battle to occur. These hostilities may be what is alluded to in the references to Cawrnur. The Pen. 51 version of triad 51 tells us that Idawc ap Nyniaw was called Idawc Korn Prydyn from which we can conclude Iddog Cordd Prydain is the same person as Iddew Corn Brydain.
However, as Gwenhwyfar would have belonged to gen. 0, the Gwenhwyach of ByS J 51 could not have been her sister. Furthermore, Medrod ap Llew has been conflated with Medrod ap Cawrdaf. The existence of two Medrods would explain why different personalities have been ascribed to the name Medrod.