Cassivellaunos, the first-century leader of British Celtic resistance against the armies of Caesar, is remembered in the Medieval Welsh tradition as Casswallon fab Beli Mawr. Beli Mawr, as we have seen, was the ultimate progenitor of the Brythonic ruling tribe, from whom a number of the most powerful royal houses in the British Celtic West claimed some kind of descent. However, there is no definable historical figure from Britain of the Late Iron Age with whom we can readily identify this figure. The origins of Beli Mawr, on the surface at least, would appear to be in mythic rather than historical reality.
Some have argued, along with the great Ifor Williams, that the name of Beli Mawr relates to the Gaulish god known as Belinos or Belenos. Numerous inscribed dedications to Belinos have been found in continental Celtic areas, especially around the Cisalpine Gaul area of Northern Italy, suggesting that this was one of a handful of meta-deities in the Celtic world to have gained pan-tribal, regional popularity. Frequently thought of as a solar figure, the name has been related both to Gaelic elements bel- / bile- meaning ‘fire’ ‘light’, as well as Celtic-derived Latinate words denoting beauty: bellisima etc. In view of this etymology and the evident popularity of this god in the Late Iron Age West, it is conceivable that this figure was somehow related to the Celtic festival of Beltain - which was celebrated in early May, when cattle were brought out of their winter quarters, and driven between two fires before being released into their summer pastures.Furthermore, there can be little doubt that Belinos was powerful cult figure in Late Iron Age Britain as well: and it may even have been the case that (like Beli Mawr in Medieval Wales) Belinos had mythical importance as the divine progenitor of the ruling tribe. The name of Cassivellaunos has been interpreted as ‘Lover (i.e. devotee) of Belenos’. The tribal grouping led by Cassivellaunos were subsequently known as the Catuvellauni ‘The Host of Belenos’, by the first century AD. One of the subsequent leaders of the Catuvellauni, who died shortly before the Claudian invasion, was known by the name or title of Cunobelinos (Welsh Cynfelyn) - ‘Hound of Belinos’.
However, much as the identities of Beli Mawr and Belenos may have become conflated, it seems unlikely that their names were etymologically related. Following the regular sound changes which caused the British Celtic tongue to mutate into the medieval language of Welsh, Belinos might be expected to produce a form like a form along the lines of Belyn or Belen. Indeed, this name is found in Welsh sources, both modern and medieval. A certain Belen o Leyn is described in the 62nd triad as the leader of one of the ‘Three Fettered War-Bands’ of the Island of Britain. The popular Welsh name Llywelyn, evidently contains this name in combination with that of another powerful Brythonic deity: Lleu (< Lugus) and Belyn (< Belinos). So if this is the medieval survival of the Gaulish god Belinos, it seems we must look elsewhere for the etymological origins of Beli Mawr.
The most convincing Gallo-Brittonic exemplar for the Medieval Welsh Beli is the name Bolgios, who is described in Ancient Greek sources as a warlord from among the Galatoi (or Keltoi) in the third century BC. More of this figure and his association with the Sack of Delphi in 279 BC will be discussed below. But what must be initially made clear is that it is the name of Bolgios (rather than that of the Gaulish god Belenos) which, following all the regular sound changes of the Common Celtic language would have rendered a name like Beli (Mawr) in Medieval Welsh.
A further connection is discernable between this name, and that of the tribal name of Belgae: the final wave of Late Iron age settlers who would have represented the last dominant British Celtic power before the arrival of the Romans. Although the precise details of the sound changes in Proto-Celtic and Common Celtic languages between 279 and 50 BC remain far from clear, the derivation of the tribal name of Belgae from the genitive form of the name of Bolgios would be entirely consistant with what we know about the Celtic inflections, sound-changes and onomastics. Thus Belgae (< *Bolgioi) can be understood to have had the literal meaning: ‘(The Host) of Bolgios’. The connection between the first-century Belgae, that Gallo-Brittonic hegemony of north-western Europe and the host of the Balkan Celtic chieftain some three hundred years before is reinforced by some startling correspondences in the archaeological and numismatic record. What this implies for the provenance of the Belgic people, and hence for the tribal-historic traditions of the Medieval Welsh, will be considered in more detail in the section after next. But first we must recap on the situation in Britain in the Late Iron Age, which witnessed the ascendancy of Belgic power over the British tribal scene.
The connection between Beli Mawr and the Belgae was first given proper academic strutiny by John Koch in 'Brân: Brennus: An instance of Early Gallo-Brittonic History and Mythology' Cambridge Medieval Celtic Studies 20 (Winter 1990). This line of enquiry has been further developed in my own The Four Branches of the Mabinogi (2005), in which the (pre-)historic origins of other Mabinogi traditions are considered. Some sections of this book are available online, including three that are relevant to the question of Belgic tribal historical tradition and its continuity in Medieval Wales and elsewhere. The first of these, The British Gog and Magog looks at the treatment of this tradition by Geoffrey of Monmouth. Another two,The Volsunga Saga and other Germanic Analogues and The Gold of Tolouse both consider the possible development and legacy of a related Romano-Belgic tradition, and its subsequent influence on the Volsunga cycle.