Related quest (but not required): Over the edge, Reaper's March.
When you approach the troll that eats a horse carcass under the bridge in Arenthia (near Wayshrine), Reaper's March, he will totally disappear. The troll will reappear when you move away from that place.
Also when you are at the horse, there's "Examine" mark, but hitting "E" does nothing. The whole quest seems strange, because the soggy bag items and quest arrows will also disappear/reappear based on the status of a troll, whether he's invisible or not.
Maybe I'm missing something here, but this quest & place does not look right. Pure trolling.