I'm about an hour down the road from Randolf Utah. Which frequently is the Coldest spot in the lower 48 states.  I've passed through there on my way snowmobiling at temps well below 0 f.  I see both horses and cows standing out in open fields.  A neighbor of mine grew up in that town. He said when it was  -40* they worked hard to get enough feed into their livestock to keep them warm. No barns, no run in sheds. Maybe a row of trees as windbreak along the fence lines. They can adapt. Another friend of mine moved out into the Unitah Basin of NorthEastern Utah. They have a lot of sub Zero temps in the winter.  Again his horses are just out in a 40 acre field. You just have to throw them lots of calories during cold weather. Find some way to keep water in them. You will need a tank heater.

I live in Northern Utah. We get a week each winter where the day time high is Zero temps. And the nights may drop to -10 to -15. My horses have never seen a barn. They do have a 3 sided run in shed. But they seem very happy to stand out in the snow unless it is blowing. They seem to use the shed more for shade in the summer than to get out of a storm in the winter.

If you will build them some run in sheds and/or barn they should be fine. Folks back east get some Ice storms which we don't get here in the west. So we don't have to worry about livestock falling down on ice. I see people posting about have to lock their animals up for a day or two until the ice melts.

The biggest problem with the cold, is not the horses surviving. But me wanting to get on them and ride in it.