Well I know its a scam but from what I have been told people have used sulfur of flowers and vaseline, it makes their horse's hair grow 4-6" a month.
It also helps if you don't groom your horse's tail as much, and use a brush like this:
Any thoughts? Have you tried it? I've heard many Gypsy Vanner owners use it on their horses to make their feathers/manes/tails grow.