Maybe... or maybe not. If it isn't good enough to be a courier horse but only good enough to pull a cart then it would be doing so for war effort simply by pulling carts with supplies to the front line. As I wrote we can only assume. Just as much as we can assume why it was left on the market. It might be as you wrote, it wasn't good enough for anything than pulling a cart. Or maybe a army lad couldn't buy all the horses because of the lack of money or room in army stables. Or maybe it wasn't mature enough for the market and war effort. Or maybe it just wasn't trained yet. Sadly, we can only assume.
For a fast travel time it won't be much of a problem, because it can be easily increased. But traveling time by foot on the maps won't change unless either game time is decreased by at least a half (bad idea in my opninon, it's already fast enough) or we get additional maps between those locations, thus increasing traveling time. Yet, those new maps would have to be filled with some content.
Only those undead knights would know. I think they just were hiding thier stuff in random locations all over Darkwood to have hidden caches from which they quickly replenish thier supplies if needed during war or just were hiding them from enemies, because they were not capable of carrying all of it.