Sub-Roman Britain (Cavalry etc)

[Agraes]

Several points can be precised.

Firstly, Armorica is NOT Brittany. It never encompassed the same definition before high middle ages unto our times. In Cesar's time, Armorica was the region of Gaul bordering the Channel up to the Belgae. It applied to a confederation of tribes. In late roman times, Armorica seems to have been the name of the provinces of 3rd and 4th Lugdunensis (spell?). That was roughly the area between Loire and Seine.

The Alans were settled in the Loire valley and so were in or just near Armorica. The rebellion could have took place anywhere in this vast part of Northern Gaul. The Britons were very active in this whole area aswell, they had settlements in the Loire valley too and what is nowadays Normandy. But the colonisation was way more important in the western part of Armorica, in the civitas of the Osismi and (more certainly in the 6th century) the civitates of the Veneti and Curiosolites.

Secondly if you absolutely need a foreign explanation on the development of light cavalry tactics in Brittany, look at the Maurs. The Maurs were famed for their light cavalry, using hit-and-run technics with javelins.

When Magnus Maximus fought the legitimate emperor Gratianus in 383 AD, the guard of Gratianus was made of Maurish cavalry. The Maurs deserted and joined Maximus british army. They became his elite guard, and a good deal of them were executed by Theodosius forces along with the usurpator in 388 AD.
A few decades after, we found the Mauri Osismiaci and Mauri Veneti protecting the Tractus armoricanus respectively in the civitates Osismi and Veneti. It's not clear if they were already there before Maximus usurpation, as there were already forts and walled towns from the late 3rd century onwards.
But to me it's possible that those units were redeployed after Maximus defeat to protect the Tractus armoricanus, and that they included Britons. Their shield emblem is celtic (even if it looks much like the Tao design of the Yin Yang to a modern eye). Possibly the romano-maurish cavalry tactics thus passed to the Britons, or Bretons...