I use Valerian root you can get it at any drug store or the mart store. It only takes an hour to kick in and lasts up to 5 hours, it does not make them dull just more relaxed. You only have to give it when you need it. You would have to experiment a little to find the best dosage. Start with 2 and see how it goes if you do it at home you will be able to tell if it having an affect. I gave one of mine 5 once when he injured himself and I had to clean the wound and he was not cooperating, he fell asleep while I was cleaning him up.
I gave my daughters pony 2 valerian root before a Christmas parade last year, it was their first parade and pony who is usually a bit reactive was just as quiet as could be.