Honestly, I've always used Suave shampoos. Cheap, and I can get the apple smell, which I find ironic on a horse. None of my horses have had an allergic reaction to it (though my dog did, so I know to keep a close eye when using people shampoo on animals).
We used to use baby shampoo on the racehorses. It was on those afternoons that the boss would wander in and find every single one of us in a horse's stall, with our noses pressed against their necks and inhaling for all we were worth. He used to joke about our "drug habit".
I LOVE USING DOVE!!! It even makes their sweat smell good :) and Old Spice Swagger body wash for our boys who do not want to smell like a girl! HELLO gotta watch out for them they want to smell good for the ladies :) those two are my favorites :)