kat393--
We have had many horses over the years, and honestly, the best way to get a white tail is to use a shampoo called Quick Silver. It is purple, and will stain your hands so wear plastic gloves, but does wonders. You can also add laundry bluing to shampoo and that works pretty well. For general purpose shampooing of a horse, Ivory dish detergent is good too.
I have used my natural soap on my horses too, but do not like to use in on manes and tails, probably for the same reason I do not like shampoo bars....just don't get the results I am after...
You can see my current show horse in my profile pic--she has a fair amt of white on her
Hope that helps!!
Susan