Stop saying that it's unrealistic when you've never ridden a horse in your life!
Yes, horses can be trained to come when you call. I know some very nice mares who will come when I call their name, and I know others who come to their owner's whistle (they're almost always trained to a distinctive whistle). Also, a horse trained to stay, WILL stay. If you put a knot in the reins on a horse trained to western saddle, it'll pretty much stay where it's put. You may also assume that you've tied your horse to a shrub that just hasn't been rendered.
On the other hand, almost no one does this with studs (stallions) -and for good reason!
You do NOT dismout a stud and leave him be. You do NOT try to make a stud come when you call. You do not do this because while most horses have a "flight!" response to anything they don't like, studs have the "FIGHT or flight" response -and often choose FIGHT! They might like YOU just fine, but they might not like your friends, or that tree beside them, or some other horse, or an enemy! And, just maybe, he might REALLY like a mare that somebody happens to be riding! And a wild, horny, andgyr horse with all five legs flailing will cause a HELL of a mess in the battle lines.
Everybody's battle lines.
So, I would suggest the horse whistle an option, but only for sumpter horses, saddle horses, step horses and (possibly, but probably) hunters. Coursers, Warhorses and Chargers should not have this option avalable -and
seriously, if you'r riding one of them, you shouldn't be dismounting on purpose in the first place.