pendragon
Also capitalized as Pendragon: a title assumed by the ancient British chiefs when called to lead other chiefs: chief war leader, chieftain, dictator, despot or king.
[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.
Where it comes from
From Middle English Pendragon, borrowed from Welsh pendragon (“chief war leader”), from pen (“head; chief; principal, supreme”) (ultimately from Proto-Celtic *kʷennom (“head”)) + dragon (“dragon; commander, war leader”) (from Latin dracō (“serpent, snake; dragon”), from Ancient Greek δρᾰ́κων (drắkōn, “serpent; dragon”), possibly from δέρκομαι (dérkomai, “to see, see clearly (in the sense of something staring)”), from Proto-Indo-European *derḱ- (“to see”)). Compare Late Latin īnsulāris dracō (literally “dragon of the island”), used by the monk Saint Gildas (c. 500 – c. 570 AD) in De Excidio et Conquestu Britanniae (On the Ruin and Conquest of Britain) as an epithet of Maelgwn Gwynedd (died c. 547), the king of Gwynedd.
Adapted from the Wiktionary entry pendragon (CC BY-SA 4.0).