Currently horses in stable get shown pretty much randomly, and it's really inconsistent.  I've included 2 pictures. One where I didn't have any horse taken out, and one after I took a horse out. As you can see the horses get sorted completely differently. I personally don't really care how the horses get sorted. It could be per tier, and then by speed or something. As long horse list doesn't get completely reordered once you do an action in the stable.

Before	After