Now that is so funny - not because the horse is stuck, but the fact in my game last night I had a similar bit of SHOCK with giant lag all of a sudden - and like you I have learned to go look for stuck spawned creatures - well to my dismay it was not a stuck horse, but two spawned Tourist and both were extremely frantic and apparently hungry. Tourist, can you imagine? Apparently for some reason that area is spawn central. Oh and they were not quite where your horse was, they were more near the center of that area - to the left of the tree.
Just like your experience I removed them from their prison and all was peaceful in GB again.
I played at least another 3 hours after that and didn't have it happen again. I also noticed the white horse decided it didn't care for downtown any longer and has joined a group of 3 horses over near the waterfall and now has herself two little foal that are white just like she is.
