Kuger is correct, it is a horse harness buckle cover, also called a buckle-shield.
But recent dating-research has proved that the LiveAuctioneers website is incorrect, it is from later than the civil war era. A US Patent (#222,720) was awarded in 1879 to D. Mosman for the basic form of the buckle-shield. Later evolutions of that basic form (such as the heart-logo style in the LiveAuctioneers photo) are shown for sale in 1890s Sears & Roebuck mail-order catalogs.
Note "Figure #2" in this scan of the 1879 Patent diagram, showing how the buckle-shield worked.