If I hadn't read creskol's description, my first thought would've been a horse rosette with a missing face plate.
"Horse owners have adorned the bridles of their horses with decorations known as bridle rosettes for thousands of years. Not only decorative, rosettes served the functional purpose of securing the bridle headstall. They also revealed important characteristics of the horse’s owner. In ancient and medieval times, bridle rosettes were made of metals, such as iron, bronze and copper. As industrialized manufacturing flourished in the 1800’s, bridle rosettes were created with glass domes enclosing hand-painted artwork, dye cuts, pictures and photographs.
Bridle rosettes identified the military war horse, adorned the bridle of a horse carrying its rider to a funeral or wedding, advertised businesses and fraternal organizations, served as ornaments depicting favorite activities of the time such as hunting. Each bridle rosette tells a story of its owner and the historical era from which it came."
Dave
Thank you, I will research rosette now, but maybe the slider is good suggestion also as, the decoration is on the strap frame, and flat oval disk went against the horse, just chancing my arm here, as i really don't know