They seem to run in packs at night to hunt, but I've never seen them in packs during the day around here (and I see them lots).
They rarely bother livestock here, other than sheep. They don't even seem to go after calves, unless one is sick, but we do have lots of deer for them to feed on.
They will definitely take out dogs, though. One of our family dogs was killed by coyotes when I was a kid (and she had another dog with her that managed to escape).
I wouldn't worry about your adult horses. However, if the coyotes are hanging around your yard a lot and have completely lost their fear of humans, you might want to take a shot (assuming its legal).
In very rare cases they've went after children. In eastern Canada, they've even went after adults, and even killed a young woman in the Maritimes > Coyotes kill Toronto singer in Cape Breton - Nova Scotia - CBC News
However, this is extremely rare.