Your find is a solid-cast brass US Army horseharness "Great Seal" rosette, dating from the very early 1900s through the end of the widespread use of horses in the army, in the late-1930s. However, this type of rosette is still in service today when the army uses horses for Ceremonial purposes (such as parades and memorial ceremonies). Yours originally has a solid cast brass staple-shaped bar on its back, for attachment to the leather harness straps. See photos below.