I parked my paint-horse (the one I got in the beginning of the game) outside a ruin, cleared said ruin, and when I came back out the horse was gone. What gives? Clearing that ruin was pretty painful (lots of trolls, I'm lvl5 and their regeneration is brutal) AND I got a Ring of Nighteye - which I've wanted for a while -- so I don't want to reload.

Do horses get attacked by random monsters if I leave them outside, is this a bug, or what?

the horse could have been attacked but I am pretty sure it started it's way back to waynon priory stables. It is definately not a glitch. you should do the Dark Brotherhood quests bacause about halfway through the quests, you get Shadowmere which is very fast and cannot get killed.

ya, the horses can be pretty unreliable in that respect

shadowmere will go out of its way to attack shit... its pretty sweet