Serpesil or reserpine used to be the drug of choice in hunter barns - called the "liquid lunge line" or how to turn a hot TB into a show hunter without all the boring flatwork. Its effects lasted from 14 - 28 days. The AHSA developed a test for it, tested a lot of horses and banned a lot of big name horse people back in the late 70's. It's not marketed for equine use now, but I do know a vet got a hold of some for me to use on a neurotic mare that was threatening to reject her foal. It is still manufactured for human use.
I suppose it's possible someone could drug a horse with reserpine for sale and the horse would appear a lot quieter than it actually was. I don't know for how long the drug would have to be out of the horse's system to not show up on a drug test, and the drug test has been around for decades.