Pendragon

[I]n the reign of Henry the Second, a body happening, by chance, to be dug up near Glastonbury Abbey, without any symptoms of putrefaction or decay, the Welch, the descendants of the Ancient Britons, tenacious of the dignity and reputation of that illustrious hero [King Arthur], vainly supposed it could be no other than the body of their justly-boasted Pen-Dragon; and that he had been immured in that sepulchre by the spells of some powerful and implacable inchanter.

The regal dominion, then, held by the sovereigns of Britain in these ages seems most properly described as an elective monarchy made hereditary; or, to take the converse of the proposition, as a hereditary monarchy requiring to be confirmed at the succession of each monarch by a popular election. […] There was this essential difference, as now in the case of the kings paramount, or pendragons^* of Britain, traces of succession by descent are noticeable. […] Thus we find in [Julius] Cæsar’s Commentaries, Gaulish Wars, vii, 4, that Celtillus, a pendragon or leader general of the Gauls, lost his life because he had endeavoured to change his delegated power into an “imperium,” that is, to make it more permanent and settled. [Footnote: ^* It is common to call the sovereigns of Britain, like Cassibelan, Uther, Arthur, and others, ‘pendragons,’ because Uther Pendragon, one that was famous among them, was so called.[…]]

At last he got his breath and answer'd, 'One, / One have I seen—that other, our liege lord, / The dread Pendragon, Britain's king of kings, / Of whom the people talk mysteriously, / He will be there—then were I stricken blind / That minute, I might say that I have seen.'

The red dragon had already been in intermittent use for centuries as a device for a general, a pen-dragon, a 'head-leader' in Celtic, and it was strongly associated with the name of Cadwallader, the last Celt to rule Britain.

To recap, with a view to understanding the 'Uther' in 'Utherpendragon', Emrys, the first Pen Dragon, fought the Angles in the 550s. Gwenddolau, the second Pen Dragon, fought the Angles in the 560s and early 570s. Maelgwn became Pen Dragon after Arderydd in 573, although by then there was no organised force for him to take over. As there were only two Pen Dragons of moment, it would have been natural for people living hundreds of years later to refer to Emrys as Pen Dragon and Gwenddolau as the other Pen Dragon.