I just came across a dead bandit marauder, an unlootable ash pile and a white horse on a road just east of the twilight sepulcher. I have the option to ride the horse, as in not steal it. I'm just wondering what npc must have died here? Are there random bandits riding horses on the roads? I've never seen that happen. Does something ride horses that turns into an ash pile when it dies?

i believed that Ashe pile is a sign that it was once summoned through mean of necromancy. Now it is dead and rendered into ash pile. So yeah, it is unlootable. for all ash pile itself. but you can loot any of values in the pile. if they possessed something previously.

Something similar has happened to me too- dead bandit with a horse. Maybe game just gives you a horse from time to time if you wander around on foot, dunno. Maybe it was same encounter.
Anyway, that horse was dead in three minutes as I ran into pair of bears, heh