Your find is specifically called a "buckle shield." As Duggap indicated, its purpose was to cover various buckles on a horse's harness which might get snagged on bushes, vines, and even the horse's long tail-hairs (see the text in the advertisement image, below). The specific version you found was invented by David Mosman of New Britain CT and was US-patented by him in 1879. Mosman buckle-shields which were manufactured during the US Patent's 17-year lifespan were marked with the Patent's date, Dec. 16, 1879. See the photo showing a marked one, below. Also, here are the original US Patent issuance form, and its descriptive diagram. The fourth image below is a scan from the 1895 Montgomery-Ward mailorder catalog, selling these buckle-shields to the public. Note, the catalog miss-spells Mosman's name.
As proven above, despite what some Ebay sellers claim, these buckle are not Confederate, not from the civil war, and were sold for Civilian usage, although it's likely that the US Military did use some of them, because buckle-shields are simply generic horse-harness equipment.
