Friend I talked to today who used to raise some high dollar Quarter horses got turned in once by a do-gooder for letting them stand outside his barn in his yard once on a cold day (the barn WAS blocking the wind)... . Vet said he already knew he wasn't concerned before he got there... he knew this farm, but because of the regulations he had to come out and check it if somebody complained. Vet told him that the best place for a horse is OUTSIDE.....they can NEVER be in as healthy of an environment in any barn as they will be if ALWAYS left outside. Now, if you insist on "keeping them warm", (put up in a barn anytime it's a little nippy, and blanketed if let out, etc.) they'll not develop a proper winter hair coat, and then they can potentially get chilled... Same vet told him that if he sees a couple inches of snow on the cattle's backs in a snow storm, he knows they're healthy and fine generally. If it's melting off of them, then he knows that they're not coated up for winter with enough insulation on their hides.
Drawback to those nice long winter hair coats though.......... it's hard to get a shock through them to the animal from the electric fence! I've got calves right now crawling under my breaker wire when unrolled hay grazing with a hot and a ground wire dragging hard across their backs with no reaction... fence has 5500V on it, no amp loss, and if I bring those two wires together it'll shoot an arc between 'em. Good news is warmer weather is on the way!!!