Shadowmere won't wander off, so there's some other hidden voodoo going on, at least for him. I know that stolen ones will wander off back home but that's expected.
I had a quick peak at Shadow and the DB last night ... But I couldn't see any control mechanism for it. From the quick look I had it looked like a horse that was standing about, doing nothing.
Just for my sanity, more than anything else, is it involved in a Quest anywhere, Arth?
(It's Use Info just details the cell it is placed in)
Mount appears to be something controlled by hard-coded means? There's no Activator, the Horse-Actor is an activator ... but I don't find any switch, anywhere, to control that functionality. I guess not having them speak means they have some secondary activation mechanism?
So very strange, and I would agree that something hard-coded is going on, somewhere ...
Horses (mounts?) are definately weird, when you consider they are owned like a Sword, but are an Actor:
setOwnerActor
http://www.creationkit.com/SetActorOwner_-_ObjectReference and http://www.creationkit.com/GetActorOwner_-_ObjectReference
(So could we do slaves, and what would that mean if we did!!? Gotta be worth another test ...)