I have a 17 month old TWH filly. She is underweight, but not extremely skinny. We live in Oakland, MD (Garrett County) so it can get pretty cold out here. My horses are turned out 24/7 with some trees and a run-in shelter. I would like to get a blanket for her to be sure she's warm when it's really cold, but I'm not sure what weight would be best. My biggest concern is when it's rainy and cold. Any advice?