also Rigothamus, Riotimus
Celtic base: "high king" rig- "king"; thamus "highest"
fl. 470 CE
Possibly an early model for King Arthur. The name seems to be little more than a title meaning "high king" like the Irish term ard ri. He seems to have flourished during the end of the Western Roman Empire, which coincides with the beginning of the Arthurian era.
Riothamus is mentioned in two Latin works, a letter to him from Sidonius Apollinaris of Lyon, and Jordanes's Origins and Deeds of the Goths. In Sidonius's letter, Riothamus seems to be a leader, one who can settle a dispute between a slave owner and the "Bretons"--however, given the date of 472 CE, they are probably Britons, not those of Brittany.
In Jordanes's Origins, the Emperor Anthemius called upon "the Brittones" for aid; "Riotimus" answered with twelve thousand men, sailing across the sea to fight Eurich, king of the Visigoths, who was attempting to take Gaul. Riotimus and his men were defeated before the Romans could give aid, and the Britons fled to the Burgundians, who were allied to the Romans. (Eurich would also later imprison the afore-mentioned Sidonius for his pro-Roman views.)
The story of Riothamus's sailing to defend Gaul is thought by some, such as Geoffrey Ashe, to have inspired Geoffrey of Monmouth's story of Arthur's taking of Gaul from Rome. However, the role of Riothamus and Arthur are completely different. Moreover, it is difficult to say whether we can identify Riothamus with Arthur, but it is possible--Riothamus's power seems centered around 470 CE, while the Battle of Badon Hill, traditionally connected with Arthur, took place ca. 500. While thirty years seems a long time, it is possible, though not necessarily probable that we are dealing with the same figure, or a figure whose time overlapped with the other. (For another possible model of Arthur, see Lucius Artorius Castus.)
Jordanes. The Origin And Deeds Of The Goths. translated by Charles C. Mierow. URL: http://www.ucalgary.ca/~vandersp/Courses/texts/jordgeti.html 9 Feb. 1998.
Sidonius Apollinaris, Letters. Tr. O.M. Dalton (1915) pp. 63-86 ; Book III. Found at: Early Christian Writings. URL: http://www.tertullian.org/fathers/sidonius_letters_03book3.htm