Well, I was thinking that it could be handled the way Fallout 2 handled the car. Meaning that on the map, the horses would be a stationary object (you dismount to explore the area). But when you leave the map to travel overland to another destination, the game would ask you if you wanted to mount the horses and go there by hoof.
Horses can't get into dungeons, but all dungeons have an entrance at ground level.
Town/village inns could have a stable for you to 'park' your horses (for a small fee).
If you leave your horse on a map and travel by foot to another location (say from Druid Grove to Windspear Hills) you would lose your horse (they wandered off on their own, or were eaten by wolves, or were stolen by a party of bandits or adventurers).
but of course all this is wishful thinking. But think of all the loot you could carry if you used them like pack mules...
