Eqyss Microtek Shampoo should take care of it. I believe Cowboy Magic makes a dandruff shampoo for horses also. You could try a human shampoo, but horse hair has a different pH level than human hair and it will most likely cause your horse's hair to become very dry and brittle.
Sorry guys I disagree with you, MTG WORKS!!! I used it on my arabian and no more dandruff, he had dandruff really bad on his mane, I put mtg on it 2-3 days a week and after about one month it is gone, and has not came back its been a few months.
I would wash the mane once a week with Head & Shoulders shampoo, no conditioner, and mix up a bottle of Listerine Mouth Wash and a bit of water and spray that directly into the mane every time you groom. Works wonders!