Sub-Roman Britain (Cavalry etc)

[Carvettia]

I now realise that the "hue of swans" part was poetic license, but perhaps the shagginess of the fur is a better lead.

Two thoughts arise from that then.

EITHER - 1 - the battle took place in winter and the horses were in winter coat (in which case, all of the cavalry horses would have been in the same condition, so why mention it).

OR - 2 - you're looking for a type of horse that has a lot of mane and tail, and hair around the feet ("feather" - known originally as fet-locks, or foot locks). Modern northern breeds with these characteristics to a greater or lesser degree are the Fell, the Dales and the Highland. All of them still can be found in grey (white). The other northern breed however, the extinct Galloway whose blood still flows in the three breeds above, was better known for being bay (brown). None of these are "shaggy" all over in summer coat of course.

And, as i pointed out earlier, these "breeds" did not then exist as "pure" breeds in the modern sense. We are talking about ancestors, just as we are with the soldiers.