Y'all are correct, it is a decoration made for use on leather or a thick fabric. The major ID-clue is the "form of attachment" on the object. In this case, the attaching-method is two identical fold-over tabs of the back of the object. The tabs are fitted through small slits in the leather or fabric, and then get pressed down to crimp the object permanenetly onto the leather/fabric.
Here's a similar brass "decorative" object, except that it has three pressed-down tabs as its attachment-method. It is a horsegear medallion, and although this version is sometimes called a rosette, it technically isn't actually a rosette.