I've had horses all ways. Where I live, it gets to -35 degrees Farenheit. About 4 years ago, the horses used to get locked up in stalls every night. They did get discharge sometimes, probably a cold or something, but nothing serious. When they are kept inside at night, their hoofs are easier to trim, and their coats don't get AS long.

The lady I bought my current horse from, kept the horses inside all the time, except for a couple hours a day when she let them out with blankets on. The horses didn't grow long coats at all. They didn't seem to have any health problems, but it was hard to adapt to a different life style once he was out all day without a blanket.

Now the horses have an open door. They can go in and out as the please. Their hooves are a lot harder to trim, but they don't grow as much either, so it kind of evens out. I like the open door the best. They spend most cold nights outside, but then if a snow storm comes during the middle of the day, they can come in if they want.