I'd just wrap the rope around your horse's neck and try a regular granny knot type knot... it's what we used to do on the riding school horses on pub rides (with head collars under the bridles)
You make the loop (loop 1) for the horse's neck, fold the rope back on itself (creating loop 2), then start wrapping the end of the rope around the 3 bits of rope towards loop 2 and stick the end through loop 2. It can take a few goes until you get the spacing correct.