I've seen the aftermath of dogs. They can do ALOT of damage. Once sat on my porch trying to call the owner of the cow feild next to me. All I heard for hours was a mamma cow mooing and a baby cow crying and bogs barking. I finally got hold of the owner but he found nothing. The next day we left out house and found the baby calf, still alive. It had one eye, it's jaw was ripped off and missing, while it's tongue hung out half checked off, along with it's ears and it's tail. They can do ALOT of damage. They did this to the baby cow WHILE the mother tried to chase them off, so I do not see them having a problem with a lone horse... Although from the pictures I cant tell, but I'd expect to see bites on it's stomach. I dont see how they would have gotten the ears without causing it to go down or drop it's head. They would stay away from the legs obviously. And altho they would go for head first, they have to tire ot down first, so they will usually bite it to get it to stay in one place and fight itself out instead of running. Altho this will help tire it, it also tires the pack, and possibly open it up to escaping.
Angels are people who can see and feel the pain in all creatures... -myself