How old is the puppy?
I'm lucky to have Tilly, she's a puppy, but she listens well. When I have to scold her, usually a NO! works fine, but if I see that she is doing something wrong, like has one of my son's toy's in her mouth, I'll go up grab the back of her neck with my thumb and index finger, grip slightly, enough to let her know that I'm there, then I'll shake and say NO! Mind you, it's not hurting her, I know she's endured much worse from my other dogs.
Another thing, if your dog is old enough, try using a shock collar. Yes, people's views on this method of training is sorted, I happen to be all for them. We have two for our two larger dogs, and when they are wearing them, they know it, they act like they actually know something. They are obeying my commands, listening better, and showing more manners.
You can also try to 'claim' your horses from Milly. Tilly likes to jump up and grab the jowls and ears of my yellow lab, she'll get on them, latch on, and shake. It's painful to the other dog, so we had to 'claim' him from Tilly. When you see that Milly is terrorizing your horses, go out and intervene by putting yourself in between Milly and the horse, and keep that position until Milly backs down (walks away, or lays down). You might have to maneuver around because Milly might run to different horses and so on, but I think you get the idea.
ETA: You can also try pinning her down on her back to assert your dominance. Let her know who's boss. We do that with Tilly every now and again, just to let her know that we're boss, we're the leaders of the pack. Pin her down, and don't let her get up until YOU are ready. If she gets up, and you didn't want her to, pin her down again.
Good luck! Keep us posted.