I bought a horse once, for no less than 1000 gold. It was a decent horse up until that first battle. At this point I realized I have absolutely no combat abilities from atop my steed, so I dismounted and and began to firebomb my enemies.
During this once sided battle that lasted all 30 seconds my horse found a way to die.
Since then I've never purchased another horse and I don't have any plans to. And now I wonder "Why would I want a horse?"; they aren't much faster than just walking to your destination with fast travel and that carriage driver outside of the cities.
If there is some reason I might need a horse how can I best keep them alive long enough to get my money's worth?