Horses everywhere in Whiterun and none of them are mine! I'm assuming I installed something that caused this to happen, but I can't figure out what script or mod is causing this. Does this look familiar to anyone?
you could try disabling mods one by one and see if the horses disapear. However the horses may or may not be added to your save file (im not sure).
Alternatively I would try and narrow it down to say 5 or so suspect mods... and then go into tessnip and look at what the plugin has added. anything indicating horses especially if added via leveled list or spawn point would be the culprit. you can find tes snip here http://skyrim.nexusmods.com/downloads/file.php?id=5064
It is caused by "WiS IV - Normal Mode & Plugins.esp", which is a truly broken mod imho. It will also change a number of in-game descriptions into Italian, LOL.