Yes I do think Valerian will work as a calmer.Its for nervous and stressed animals.You could combine it with Skullcap which also helps calm horses down. I think you can use it until the horse calms down and then continue to feed it until you feel the horse has calmed down. I'm not sure though maybe be safer to look up the internet Hope it helps
It's supposed to be good, but I think I read somewhere that you can't give it to Arabs, they react badly to it. Hopefully you haven't got an Arab (or it's just a myth!).