We have 3 packs in the area. Once in a great while they will all come together, and their meet-up point seems to be my neighbor's yard. The horse fence we have around wouldn't stop them, but it probably deters them from entering our yard. There is a wash that goes within 20 feet of our corrals. The mare mostly ignores them. The gelding lived in the open on ranches, and he gets very upset. However, I can't imagine them tackling two healthy horses, or surviving if they did.
My dogs hate coyotes, but they don't try to engage them either. The older dog we gave to my daughter had spent some time around them because he used to get out and stay in the desert for 24-36 hours. He hates coyotes as well, but avoids them if possible. When the other two were pups, Dan would stand guard over them while they retreated to the back porch. I think Dan somehow got the idea across that coyotes were dangerous and not to be tackled unless cornered. At 50 lbs, the Border Collie is about the size of the coyotes around here, while the Australian Shepherd is larger - but 2 against 6 or 10 wouldn't work out so well either.
Mostly, I'll just see 2-3 moving along the wash. As long as they are just passing thru, neither horse gets too upset. The dogs will bark, but won't give chase.