A water-based or aloe-based shampoo provides one of the best mixes to bathe your blonde-haired horse. If a shampoo contains moisturizing agents that give your horse's skin the nutrition it needs to retain the coat moisture, then it is a definite must-have.  Although you can get many cheap shampoo options that contain bleach to lighten the stain on your blonde horse, put in a few more bucks, and you can give your horse a non-irritating, hypoallergenic shampoo that will remove any hard stains.