Twice in a new game I have encountered NPCs holding torches standing by their horses on roads (one was a guardsman, one a courier). They neither move nor speak. You cannot speak to or pickpocket them. If you attack them they protest but still don't move. Their health declines and they can die. Approaching the horse produces no result--you cannot mount or steal them (no icon).

I have never seen this before. Any ideas what is happening here?

It sounds like the NPC's AI packages are not triggering for some reason or other. Are you using any mods?