The following list of legendary kings of Britain derives predominantly from Geoffrey of Monmouth’s circa 1136 work Historia Regum Britanniae (“the History of the Kings of Britain”). Geoffrey constructed a largely fictional history for the Britons (ancestors of the Welsh, the Cornish and the Bretons), partly based on the work of earlier medieval historians like Gildas, Nennius and Bede, partly from Welsh genealogies and saints’ lives, partly from sources now lost and unidentifiable, and partly from his own imagination (see bibliography). Several of his kings are based on genuine historical figures, but appear in unhistorical narratives. A number of Middle Welsh versions of Geoffrey’s Historia exist. All post-date Geoffrey’s text, but may give us some insight into any native traditions Geoffrey may have drawn on.
Geoffrey’s narrative begins with the exiled Trojan prince Brutus, after whom Britain is supposedly named, a tradition previously recorded in less elaborate form in the 9th century Historia Brittonum. Brutus is a descendant of Aeneas, the legendary Trojan ancestor of the founders of Rome, and his story is evidently related to Roman foundation legends.
The kings before Brutus come from a document purporting to trace the travels of Noah and his offspring in Europe, and once attributed to the Chaldean historian Berossus, but now considered to have been a fabrication by the 15th-century Italian monk Annio da Viterbo, who first published it. Renaissance historians like John Bale and Raphael Holinshed took the list of kings of “Celtica” given by pseudo-Berossus and made them into kings of Britain as well as Gaul. John Milton records these traditions in his History of Britain, although he gives them little credence.
Source: List of legendary kings of Britain – Wikipedia