A rug that is put on the horse to protect it from the cold and/or rain when it is outdoors (in the field, not during riding). Outdoor rugs are generally waterproof. There are different rugs for warm and cold weather, which provide less or more insulation, depending on the degree of filling. Some outdoor rugs are waterproof but have no filling, so they can protect the horse from the rain without it getting too hot in warm weather.