Baby powder? I know we used it on my show goats to whiten their coats. You'd rub a large amount into their white fur, let it sit then wash it out REALLY well. Not sure if it works on horses or if it will irritate a horse's skin, that's just what we used on our goats. There's also a shampoo called Bright Lights. It's purple, and I know people at my barn used it on their show steers to lighten their coat.