Horse brasses were used to decorate the harness on driving horses, especially in nineteenth-century England. They date back, as far as we know, to the pre-Roman period, but reached their height under the Victorians. (Although called brasses, the older ones were made of bronze or even silver).
Victorian horse brasses showed all kinds of designs including crosses, animals, horse shoes, hearts, and of course family crests.
In fact, during the Victorian era, the number of heavy brasses seen on working horse harnesses were so high that animal welfare movements campaigned for a reduction, which resulted in the production of lighter brasses which were stamped rather than cast.
Horse brasses are still commonly collected today, but less seldom seen on horses simply due to the decline in working horses. This image of Shire horses plowing (It's actually a fake shot) shows horse brasses "in the wild" on the strap behind the saddle.
Image source: Malcolm Morley via Wikimedia commons.