They are called horse brasses, and are mounted on a leather strap and attached probably to a carriage or horse harness. From that point, I don't know where they decorated the horse. The English have been very fond of these, and many, many households have a strap of them hanging on the wall. 3-7 pieces per strap. We purchased some in the 80's while on a trip there. They were modern, but I am sure the originals are at least Victorian. Perhaps some of our English friends could expand on these and tell us where on the horse they were actually used. Many are made currently.
Oops--everyone else got in before me.