Gwallog ap Llaennog

Gwallog ap Lleenog (Old Welsh Guallauc, Middle Welsh Gwallawc; his father's name is spelled variously Lleinauc, Lleynna[u]c, Lleenawc, and Llennawc) was a hero of the Hen Ogledd. He has long been considered a probable sixth-century king of the sub-Roman state of Elmet in the Leeds area of modern Yorkshire, though some more recent scholarship would identify him more tentatively simply as a 'king of an unidentified region in the north'.

Gwallog is most clearly attested in a note incorporated into Anglo-Saxon royal genealogies of Northumbrian kings found in London, British Library, MS Harley 3859 (the earliest manuscript of the Historia Brittonum). These are thought to originate in a perhaps eighth-century source and so to be relatively reliable. Commenting on the regin of the Bernician king Hussa, the regnal list states

Thus it appears that Gwallog joined a group of Brittonic kings, including Urien Rheged, Rhydderch Hael and Morgant Bwlch of Bryneich, in an attempt to defeat the Angles of Bernicia. This endeavour failed after Urien was slain.

Gwallog is the addressee of two poems in the Book of Taliesin which Ifor Williams identified on linguistic and historical grounds as (in part) plausibly originating in the sixth century, and possibly being genuine praise-poems addressed to Gwallog. These afford some evidence that Gwallog was a king of Elmet. If so, he was apparently succeeded by Ceredig, the last king of Elmet, who was deposed by St. Edwin of Deira; this would be consistent with the appearance of a 'Ceretic, son of Gwallawg' in one of the Welsh Triads. However, as evidence for sixth-century historical realities, this evidence is very tenuous.

...
Wikipedia