This was the firs time that I red about Hyperboreans. Very interesting - and inspiring, to a native finn. Maybe I'll open my thought about this a little, for the annoyance or delight of you all.
It is only my personal view, but I could well see Hyperborea being based on ancient "Finland" or the cultural area, if you look at Howard's map. In that, Hyperborea is nearly precisely placed over Finland, Estonia, Latvia and Lithuania, - which all share the same cultural and linquistic roots, different from the surrounding areas. Howard's map lines cultural borders very precisely. I do not quite agree with the thought, that something like Russia would be in question. After all, Russia was established fairly late in history by Vikings, and before them, the land within Russian borders was occupied by Ugrian and Finno-Ugrian people.
"So we have a nation that is the birthplace of many tribes, many of whom go on to found their own nations. A nation beset from east and west, which has absorbed a fair amount of foreign bloodlines, and that practices a different religion and is viewed with suspicion as a result."
Finland has always been - and still is, separated in tribal areas and cultures with different customs and even liquistics. There's nothing much to add to that...it's a match.
"Within the Conan series the Hyperboreans are not presented “up front” as so many other cultures and nations are—they instead loom steadily in the background shadows."
In my mind, the "feel" of the existanse of Hyperborea as describen in this text, the mystery matches fairly well the point of view that is painted in the first legends ever preserved to these days. The Viking trade route moved past the finn-estonian coasts those days - very rarely stopping there. In Icelandic sagas it was called "the shores of fires" because of the line of the beacons that were lit along the shore as a warning signal, whenever a ship sailed past. The Chronicles of Nowgorod often describe how their conquerors, after loosing a battle, escape into the finnish woods and get killed there. All of sudden, out of the blue, the joined finnish-estonian tribes destroy the capital of young Sweden:
"they stayed quite secretly in the archipelago of Svea usually with the secret army. Once they got such a notion that they burned Sigtuna, burned it along the bottoms and the town did not get help anywhere. The archbishop Jon was killed there and many heathens were happy from it that the Christians were so unlucky. And the whole of Karelia and Russia were happy about it." - so wrote Erik the King of Sweden, in his chronicle, and never spoke of it again.
Overall, in the known history of those early days,"loom steadily in the background shadows" seems quite descriptive.
However, what does not match, would be the cavalry. I have never heard, or even thought it possible, for Finland to host cavalry in large scale. The climate is against it, aswell as the terrain. There is no flat pasture anywhere. However, on the other side of the gulf in Estonia, there is flat land a-plenty, and if you go a little further south....although outside the cultural bordes which I previously pointed out in oward's map, you find Hungary. Hungarian language is known to be the only other language in the same language family with finnish, and the two languages (apparently) share the same roots. Could we imagine, to our humor, that the Hungarian horse-lords were once connected to their linquistic brothers, as far to the north as the land of the forest-finns?
What were the Hyperborean's like?
Well, what are the finns like?
"big-boned, of slow speech and violent natures.."
Quite accurate, even if I say so myself. What doesn't match here would be the heights. Perhaps to someone from southern countrie,s finnish people would indeed appear tall in stature, but we are still short of, - for example, our neighbors the Norwegians.
At this point, I'd like to connect a legend to another legend.
In the 18-hundreds, when finnish folk-lore was finally recorded and written down from some parts, a lore-collector heard from an old blind woman this sort of a tale:
"Before we came here there lived the russkies. Before them were the Sami people, and they had conquered the land from the Yotuni."
This knowledge of the old woman had lived from generation to generation before it was finally written down, and it reached so far to the past that the end of it was in myth. "Russkies" refer to carelians - yet another finnish subroup. The Sami people still exist and now live in Lapland. However "Yotuni" from which the Sami people had conquered their land, has no one today ever seen. It refers to "Giants", or "tall people". Let our imaginations fly.