What can I do to get my horses's mane to grow back fast?

Step one is good nutrition. Of course, be sure that your horse is getting enough calories, so that she can grow hair. Then think about supplements. Biotin, in addition to helping hoof growth, helps mane and tail growth too. Now, spray or rub-ons like you asked about. I have heard people swear by a product called M-T-G, you can get it online or at a horse supply shop.