Cramp bark ought to help if that’s the problem. I’d look closely at nutrition as well. That sort of issue isn’t contagious and if more than one horse has it, it’s almost certainly diet related. I’d particularly look at mineral levels, especially magnesium.
Throw a TBS of cramp bark powder on their grain.
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.