I use Head and Shoulders shampoo on my horses.
I really don't use it that much because most my horses don't have a bad dandruff problem. I usually put a little in a bucket and dilute it with water then use a sponge to wash my horses.
I've even put some in my hand once and lathered it up and put it on my horse.
I've heard listerine works for getting rid of dandruff. There's another thing out there but I can't remember what that is.
If I don't get the chance to bathe my horse I take a dryer sheet over them real quick. It gets the loose hair and dandruff out. (:
Hope I could help!