How about normal NPC's?
If there isn't yet, there likely will be a mod that makes it so there are more travelers.
They had one for oblivion. And if that mod is in existence and doesn't use horses, I'm sure the suggestion could be made.
But also, the lack of riders makes sense. If you remember in Oblivion the stable master of Bruma talks about how they don't have great horses because the nords have no use for horses on the rocky uneven terrain of skyrim.