Two aspects of the mythologies surrounding Loth seem to have some credible historical basis, namely the idea of his origins as a Brythonic solar deity called Lleu or an historical or pseudo-historical figure called King Loth, one occasional claim seems less credible - namely, that Loth was a Pict, rather than a Brython per se.
The medieval Scottish historian Hector Boece (c.1465 - 1536) seems to have been one of the earliest to make this claim overtly, but we also find it hinted at in an oft-repeated idea in Arthurian romances that Loth was not only King of Lothian but also of Orkney. Such an assertion is to be found in the Morte d'Arthur, by the C15th English writer Tomas Mallory (c.1405-1471): "King Lot of Lothian and of Orkney then wedded Margawse that was Gawaine's mother" (Book.1, Chapter II). A more "historical" account was later offered by Alexander Shields in 1687:
Goranus the 45th king of Scots, earnestly dissuaded Lothus king of Picts to entertain the league with the Saxons, not only because they were treacherous and cruel, but because they were enemies to the country and to the religion they professed, concluding thus: Homini vero Christinano id longe omnium vinderi, etc. But to a Christian nothing must seem more grievous, than to consent to such a covenant, as will extinguish the Christian religion, and reduce the profane customs of the heathen, and arm wicked tyrants, the enemies of all humanity and piety, against God and his law. Whereupon Lothus was persuaded to relinquish the Saxons.
- Alexander Shields, A Hind Let Loose: An Historical Representation of the Testimonies of the Church of Scotland, 1687.
Intriguingly, Shields here has Loth as a Christian king, rather than the king who, according to Jocelyn's apparently historical account, was "most Pagan in his creed". However, the idea of pacts between the Picts and the Saxons against the native Britons South of Pictavia does seem to have a historical basis in the works of both Nennius and Gildas.
But what basis is there for suggesting that Loth was a Pict rather than a Briton as such? Intriguingly, there are a number of Pictish-style brochs around the territory of the Gododdin, at Tappoch in Falkirk, Calla in Lanarkshire, and Duns and Galashiels in the Borders. The broch known as Edin's Hall near Duns would seem, by the placename, to be connected with the same Eidyn who lies at the placename root of both Edinburgh (Din Eidyn) and Carriden (Caer Eidyn), and thus seems to have a specific Gododdin connection. However, Edin's Hall is not only distinctly Northern in style, but has strong similarities with that at Gurness, Orkney, forming part of a minor oppidum. Is it possible, then, that there is some historical truth in the idea of a King Loth with Orcadian or Pictish connections?