The eleventh century Old Welsh text Y Mabinogi gives us some clues about a mythological figure called Lleu, from whom the placename "Lothian" appears to derive. However, Y Mabinogi is also one of the earliest of texts which makes extensive reference to the court of the legendary King Arthur, at the forefront of the wide range of Arthurian writing which swept across medieval Europe, but particularly Britain and France. This corpus of literature appears in various languages, whether Old Welsh, Norman-French, Middle-English or Old Scots, and it presents a complicated series of versions of the original Brythonic mythology. Within this range of literature, however, we also find numerous references to an Arthurian knight called Loth or Lot, and this figure seems to derive in some fashion from the Old Welsh myth of Lleu, the mythical progenitor of Lothian.

Perhaps one of the best known indigenous Arthurian texts is the Historia Regnum Brittaniae ("History of the Kings of Britain"), written in Latin by the Welsh scholar, Gruffydd ap Arthur, better known as Geoffrey of Monmouth, around 1136-38. Here, we not only have extensive Arthur references, but also several mentions of an apparently historical king, one of Arthur's knights, who has specific Lothian associations, as well as a family relationship to Arthur himself:

In this, we have a figure called Lot, brother-in-law of Arthur and father of none other than Gawain (Welsh Gwalchmei, Latin Walganus) and Modred (also known as Mordred). This Lot is also in political control of a region called Lodonesia, the Latinate form of the Norman French Loenes/Loeneis, signifying Lothian. Geoffrey also gives the following information in describing Arthur's alleged campaign to invade the Continent:

So, according to Geoffrey, we have an Arthurian knight called Lot with family connections to both the Arthurian and Norwegian elites, who is Consul of Lothian, and later, King of Norway.

However, the Arthurian myths were also very popular on the Continent in the medieval period, not least in Norman France, and an approximate contemporary of Geoffrey of Monmouth, Chrétien de Troyes, is particularly famous for his Arthurian sagas. Significantly, he confirms Geoffrey's genealogy of Loth's Arthurian connections in his epic Perceval. In this scene, Gawain is being tested on his knowledge of Arthur's court by a noblewoman:

Although Modred is not mentioned in this list, it confirms Geoffrey's association of Loth with Gawain. Yet we find one other significant piece of Lothian marginalia in Chrétien's work, in his Yvain (the Norman French version of the story of Owein ap Urien, a genuinely historical king of Rheged who also appears in Y Mabinogi). Here, Yvain is courting a noblewoman who appears to be from Lothian (Laudunet):

In this is perhaps a remnant of an indigenous story of a royal intermarriage between the Brythonic kingdoms of Rheged and Lothian, but the specific epithet of Landuc is not explained. Nonetheless, a later series of references to an apparently historical figure with Lothian connections also occur in Welsh manuscripts which may help to interpret Chrétien's comments. According to WJ Watson:

In these sources, we appear to have a figure called Lleidun or Lleudun with definite links to Edinburgh (dinas etwin and dinas Eidin respectively), but also further possible Continental connections, this time with Brittany, another Brythonic Celtic region in ancient times, and also one which figures in Arthurian lore. And given the common scribal error of mistaking a u for an n, it is possible that Chrétien's character Laudine of Landuc is in fact the same as the Welsh Lleidun Luydauc.

One way or another, the medieval Arthurian romances nearly all point to a legendary King Loth with at the very least a political base in Lothian and surprisingly close family ties to Arthur himself, and other major figures within the Arthurian circle. In some measure, this King Loth may be a romantic rewriting of the mythological Lleu of Y Mabinogi, but he may also have an equally nebulous relation to the historical or pseudo-historical King Loth mentioned in other medieval sources.