I know this sounds a little bit different, but does anyone know if theres a holster that will hold an AR15 that attaches to a western style horse saddle?

Holsters?? NO.  You might be able to find scabbards though.  Any decent leathersmith or hobbyist ought to be able to crank one out relatively easily based on expanding a current pattern for a lever or bolt rifle.  thoise are basically sleeves without much padding or extra protection.   Usually since the scabbard ran beneath the riders leg, it wans't much more than a one end closed tube.

Or if you wanted to make some mods to it a tactical rifle case, that might work in a hurry.  You would want to be able to leave the butt end open, reinforce the zipper area so the zipper can't open past the end and then straps to hold it in close and keep from bouncing around.

Try this one

http://www.packsaddleshop.com/CatScabbard.html