Ratcatcher in the equine context is the least formal dress worn by a foxhunter. It consists of fawn or brown breeches and a tweed jacket. Colored hunting ties have become common.
Traditionally ratcatcher is worn during the first autumn hunts or "cub" hunts, when new hounds, horses, and riders are being trained. Ratcatcher is also worn on informal days. Some hunts expect new riders to wear ratcatcher - that way they can be spotted in the field and helped...or avoided...as the case may be. Juniors, especially boys, normally wear it. Juniors also wear ratcatcher when showing hunters, especially ponies, at least in the UK.
In the US, a "ratcatcher" can also be a white shirt with a stand up collar, commonly worn in the show ring.