Three letters... MTG!!!!
It really is the greatest stuff in the world. It smells like bacon and can be a bit messy (i apply it with a glove), but I haven't found anything that compares to it. It will eliminate dandruff in a day, along with fungus, rain rot and makes hair grow great in general. I use it for almost everything.
When I got sonny, his tailbone was completely full of dandruff. I literally couldn't even see his tailbone it was so bad. I haven't seen anything like it before. After a little MTG, it was COMPLETELY gone! He is good as new now :). He also gets beat up a lot on the field, so once the wounds are completely healed I put MTG on them to make the hair grow back faster. I also use it on the back of his hind legs, where it some times gets gross from where pee splatters, and it helps a ton. I really do use it all the time :) Schneider Saddlery