Are coyotes a threat to my mare and her soon-to-be foal?

Hi! I have a mare who will more than likely foal within the next couple of weeks. About 3 people, including me, were sitting by the barn yesterday at 6. A coyote came out of the woods walked up to us about 30 feet away, then he just walked away. It was daylight and I am really worried about the safety of my mare and foal. Will coyote's attack horses? What will happen if he tries? How can I keep my horses out of danger. They're boarded so I can't leave them up overnight. I don't know what to do! :'( Please help!