I found another small buckle, I was wondering if it was a piece of clothing or horse tack, and for the future is there a good way to tell the difference?
As a general rule (there are always gonna be some exceptions), buckles for clothing tend to be brass (or some other non-ferrous metal), because an iron buckle tends to make a rust stain when the clothing gets wet (for example you get caught in a rainstorm). Which is why actual military Navy buttons do not ever have an iron/steel back -- with one exception, civil war era US Navy kepi-hat strap buttons.