Staying dry is a big part of staying warm. Your horse’s coat will shed some rain unless the precipitation lasts a long time or is driven by a strong wind. In severe weather, outdoor horses may benefit from waterproof blankets—if they fit properly and are checked often to make sure they don’t rub. Blanketing a wet horse prevents the coat from drying, however, so bring your horse into a stall and allow him to dry completely before putting a blanket on him. In a pinch, you can put a loosely fastened blanket on a wet horse, placing large handfuls of straw or hay at frequent intervals between the horse’s skin and the inside of the blanket so air can circulate to help the coat dry. This is a temporary solution, and the horse will need to have this blanket removed and a dry one put on before he’s either left in the stall or turned out.
Source: Kentucky Equine Research