Okay, seeing as there is some discussion going on here, I'll add my own idea on the simplest kind of implementation I can think of that will satisfy people's immersion.
Horses/donkeys/cows/wheelbarrows can be stored in stables/barns/sheds by riding them into/near the location, and some kind of interaction (press button, presumably). There can be some kind of animation for getting off your horse when you do this.
Once it is inside the stable/barn/shed it can not be interacted with directly (maybe you must interact with an idle animation of your horse inside the stable, but you are not directly interacting with the horse, you are interacting with the inventory of the stable). Interactions such as breeding should be provided here, and depending on the number of horses or w/ever stabled there must be enough food/hay to support them (or the horses will starve/suffer stat or hp damage). You can ofcourse retrieve your horse from the stables as well.
As long as you are riding your horse or have it in the stable it is bound to you (others cannot use or retrieve it) it essentially counts as equipment (wars/sieges/thieving not withstanding).
The above is (in some way or shape) allready planned for the game, so now for the difference, when you get off your horse in the field.
The best solution I could come up with is that a static object is created (with an idle animation for living creatures) that can be interacted with by anyone. Your horse can be stolen, it's saddlebags looted, or it can even be killed, it's w/ever. Possibly some kinds of warhorse can be "bound" to not listen to anyone but it's master, but these horses could then ofcourse also not be traded.
When server down-time comes around any horses not "equipped" can have one of two things happen, depending on what the regular process for maintenance is.
1. they simply get removed from the game, along with any items they may have been carrying, it is lost forever.
2. It is reset at it's current location as a "wild" mob, carrying all the items & equipment it previously had, but now wandering around/running away w/ever. It will have to be tamed again.
Optional additions:
1. The horse wanders around/moves on it's own a bit while not mounted, meaning leaving it for a longer stretch of time could lead to an annoying search.
2. There is an ability involved with the horse-riding skills that lets you call your mount from within a certain range while not mounted. This will cause your horse to run towards where you were when you last called it, or stop when it comes within a certain range of you.
3. The horse has a ttl (time to live) after which it will go feral, regardless of server down-time (it will persist through server down time if it's ttl has not been reached)
4. Maybe stealing horses from stables is possible?(at least during wars)
P