The stalled horses are stalled under the same conditions mentioned above, or because it's ND/MN, if it's just really very cold. (ie, it was -13F this morning without windchill. Yesterday afternoon in the bright sun, the windchill was -20F).
Mine are outside. One doesn't have a blanket because she's a little furball and I don't have one her size. She has plenty of hay and a shelter though and seems to be doing fine. The other has lost weight lately, so she has a heavy weight blanket with liner underneath. They both seem pretty alright.