I just figured out a few days ago the Geoffrey of Monmouth identifed the wrong Constantine as the father of Constans, Ambrosius Aurelianus, and Uthyr (Victor) Pendragon (Arthur's father). Arthur's cousin from Brittany in the Arthurian stories is Howell, also known as Hole, and Hywell in Britain and in his native Brittany as Riwal Mawr, and I've forgotten his Roman (i.e., Latin) name.

Hueil however was one of the many sons of Caw/Caaus living with other Britons among the Pictts in Strathclyde (slightly before because his son, Gildas, the Saint and historian, said he came to Britian as a very young man and that must have been close to 500 CE from known dates of Gildas.

Back to Hueil: He evidently abandoned the British and joined the Picts, and started rising hell, so King Arthru went north and killed him (that has bee suggested among other things as the reason that Gildas's history doesn't mention King Arthur, although he must have perrsonally known Arthur. With Irish history and Irish mythology (he's prominent in 'The Tain' and the stories leading up to it) can come up with the following for yet other names of Fergus: Fergus Mor mac Eirc, Fergus mac Roich (common), Fergus, the great son of Roech, Mac Roch, and Fergus mac Roith. From English and Scottish histories I found him as found as Fergus mac Erca. That god he didn't go to Alba (as it was termed in Ireland), or we'd hav a latin name to deal with, too.

Fergus's migration to Kintyre (north, not east) was about the last of the Irish migrations to Britain and Alba or Albion (as the Irish called what we now call Scotland) Note that Irish had already taken almost the whole coast of Wales, and Cornwall to St Ives (an Irish female saint as I recall), and had already been expelled from much of it, or were currently being expelled. Cunneda and successors, starting c 430, had cleared North and South Rheged by about 500 CE, and Irish were expelled from Anglesy Island and nearby Gwent about 500. Sometime between 500 and 510 Agricola, son of Vortipor, had reconquered Demetian, but assimilated the Munster Irish there. Land locked tiny Breicheiniog (modern Brecon) was founded by Munster Irish, but knowning they were surrounded by British, they stayed on very good terms with them, and over a few centuries of intermarriage they were assimilated to death. [Bryan Brecheiniog was apparently the little King in the Arthurian romances, but not because he was small, just his kingdom.] Actually quite a few Brittish ones weren't much bigger, and that's one reason why history of the period is so hard. What do we do about all those people that are called kings, and where do we put them? I don't know how the coast of Conwall were cleared and the south cost of Wales, with it's many small British kingdoms, at the same time.