Two entries with the name Conan.
Conan
A nobleman who urged Mordred, after he usurped the throne of Britain, to reconcile with Arthur. Mordred ignored his advice.
Source
The Misfortunes of Arthur | Thomas Hughes, 1587
Conan
Nephew of King Constantine, Arthur’s successor to the throne of Britain, in Geoffrey of Monmouth’s Historia. Conan murdered his uncle and poisoned all of Constantine’s sons so that he could take the throne. After several years of tyrannical rule, he fell off a horse and died.
In his sixth-century diatribe, Gildas reproaches a certain Conan of Powys, accusing him of falsehood, fornication, adultery, and murder. This is probably the historical prototype for Geoffrey’s character.
Sources
De Excidio et Conquestu Britanniae | Gildas, c. 540
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138