It is a Morseman's Patent buckle-shield for horseharness buckles. It is called a buckle shield because it fit over buckles on horseharness straps, preventing the buckle from snagging on vines, bushes, or the horse's tail-hairs. It was invented by Mr. David Morseman of New Britain CT and patented by him on Dec. 16, 1879. See the original US Patent info scans, posted below. Also see the advertisement drawing showing it, from the Horseharness section of the 1895 Montgomery Ward mailorder catalog, below.
Ones made before Morseman's patent expired (17 years after the patent was issued) are marked "Patent Dec. 16, 1879." If yours lacks the patent marking, it was made after the 1896 expiration of the patent. See the photo below.
After-posting note:
As seems to happen surprisingly often, my buddy NHBenz and I were typing our replies at the same time, and he finished his before I gathered the Patent-info & photos and posted mine.