None of the creatures have a PC Property item on them. That's something that Bioware added when they created the horses. The PC Properties stores all the data for horses. Bioware integrated the coding into all their scripts, so that it's possible the monsters are trying to do something that somehow creates the item when it shouldn't. I thought I had blocked all of their function calls that could create the PC Property skin, but apparently not.
So, if you see one fall when a monster dies, please tell me specifically what monster it was and what you did to kill it. It could be caused by any number of actions, melee, spell casting, etc.
For example, several PC Property skins were found in the Noob Halls. I saw some on the floor, but I have never witnessed one dropping. I don't typically use spell casters, so maybe melee doesn't trigger the necessary function call that creates the item in the first place.
You should also never, ever see a PC Property skin in your inventory. If you do, please try to report exactly what you did that created it.
Thank you.