My horse stays outside year round, regardless of the weather. If it's raining, they stand under the trees. If it's cold, they huddle together. If it's excessively hot, they stand in the valley of the field, in the dug out. Winters here can get as cold as -50*C with windchill.
I will admit that I'm a little worried for my 16 year old mare. I don't know how she'll handle the coldest days of winter and I'm pretty sure she'd lose a blanket if I put it on her. If need be, I'll move her to a stall for the coldest months and then kick her back into the field when it warms up.