Logan, I'm so sorry to hear that Butter is being so mean to sweet Mimi.
I would keep them separated for now and try a behaviorist like Sallyanne said.
If Butter is not behaving by next week I could always give him or Mimi a ride to Vermont for a little vacation.
I will probably e-mail you later tonight, I'm on my way out the door to my daughter's riding lesson now. Hugs to you.