Geoffrey lists him as an early King of Britain in the third or second century BC. He succeeded his father, King Cherin, and was succeeded by his brothers, Kings Eldad and Andragius. According to John of Fordon, a Scottish historian, claims he was an ancestor of Lot and Gawaine.
It is highly likely that Fulgentius was none other than Fabius Planciades Fulgentius, the Latin mytographer and allegorist who flourished c AD 500, and who simply found his way into the Arthurian cycle at the whim of the various authors.
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Chronica Gentis Scotorum | John of Fordun, c. 1385