The "Northern theory" of Arthur's upbringing has been described in recent years as a product of medieval Welsh poets working in the Lake District and adapting their subject matter to fit the country in which they lived. Some modern scholarship has swung hard the other way and, sadly, discounts Arthur, Vortigern and the Saxons Hengist and Horsa, as mythic gods or heroes "historicised" by bardic song and tale-telling. However, the original of Arthur as a god is not identified.
Vortigern, Hengist and Horsa are all probably "honorifics" or titles, that is, they may not be real names. Vortigern means overlord or High King, Horsa probably does mean Horse and Hengist is a name meaning Stallion. Arthur is not mentioned by the Anglo-Saxon Chronicle, but Vortigern, Hengist and Horsa are, so there is a good chance they really existed. Sharon Turner in 1799 suggested almost in passing that Arthur could have been the same person as another leader of the fifth century, Rigotimos. Sidonius Apollinaris, a well educated Frenchman who became a bishop, wrote a letter to Rigotimos around 469 AD. Geoffrey Ashe (1985) maintains that of the four leaders, Arthur is the one known to us by his own, Celtic name; and that Rigotimos or Riothamus was his official title as "High King". ( ref )
However, it's true that many "Arthur" placenames have no defined links to the 5th century. For instance, the local name of "King Arthur's Round Table" for an earthwork at Eamont Bridge (Penrith) is probably an example of poetic association since the "henges" there pre-date Arthur by at least 2,000 years.
Yet there are many other placenames in Cumbria and Lowland Scotland which recall him. The persistence of the romances, legends and placenames indicates something dramatic and enduring in the character and activities of Arthur. It has even been argued from placename evidence that the Arthurian battle sites of Nennius' manuscript are all located in Northern Cumbria, Northumberland and Southern Scotland.
We don't know whether that outstanding soldier-leader really was active in the Celtic / British areas, or whether Rigotamos was the original Arthur. However we cannot doubt that the active, thrifty ancestors of the Fell pony type would have been well suited to demanding mobile cavalry work.
As for the British horsemen of 5th century Rheged, perhaps a memory still lingered of the Roman ala Augusta Gallorum Petriana milliaria bis torquata civium Romanorum, or Ala Petriana. The name "Ala" means a Wing, ie its position was out to the side of the battle formation, and possibly also implying the speed with which the cavalry could "fly" to any situation. This unit was mentioned by the historian Tacitus in the year AD 69, when it was employed in some strategic manoeuvres in Italy. It was recorded in various inscriptions in northern England. It arrived in Cumbria in AD 71 and from its title consisted of French (Gallic) troops. Originally 12 troops strong, with 32 men to a troop plus officers, it was moved to the new fort at Stanwix in AD 130, where it was to be stationed for nearly 300 years. It was then 24 troops strong (milliaria), the only cavalry unit of that size in Britain. The words Augusta and civium Romanorum seem to indicate that it had special status; Augusta, "the Emperor's Own", is a compliment applied to outstandingly good units, as indicated by their being twice awarded the "torque" or neck ring for gallantry; while civium Romanorum means the troopers had been made Roman citizens. (Bedoyere)
They appear to have been held in readiness for task-force activities in this key Border area; The local people were settled and prosperous from more than three centuries of the "Pax Romana"; they were also not allowed by law to carry arms, so they were unlikely to be skilled in warlike or violent situations. Whenever there was border trouble the Ala Petriana, like the British SAS (Special Air Service), would be alerted via the fast Roman signalling system and sent galloping to deal with the problem.
After the Army's withdrawal, the cavalry's ability to turn up in the nick of time may well have been nostalgically recounted when there was conflict or trouble. For nearly a century after the Army's return to Rome to defend thier city against invaders, the British still expected, indeed pleaded with, the Roman authorities to send soldiers once more to preserve peace in the land.
Dent talks of the Roman cavalry becoming, little by little, "the hereditary fighting caste of a wholly Celtic society". It would be entirely possible that the original cavalry of the Pax Romana passed into legend as Arthur's "knights", who "will come back to put things right".