I have had this happen numerous times...Got simple humans on sabercat mounts.(they refused to get on direwolves)..eventually, the mounts got killed...but the simple humans were "riding" around as though still on the mounts. Commanding them to dismount failed, commanding them to go anywhere, attack anything failed...the only way I got them to stop this behavior, was to use a different mod to "store" them and then summon them from "storage" again.