Looks like a horse-harness buckle, used to join several adjustable-length straps. You can see some examples in the photos below. At "about the size of a quarter" I'd say yours is from a halter/headstall (a version of bridle, shown at the extreme left in the first photo, a US Army Model-1904 headstall). A larger version is found on saddlery straps. Saddlery: the second photo shows a fully-rigged a US Army Model-1904 McClellan saddle... look at about 7:00 just below the "Great Seal" eagle rosette.
Click once on the photo, then a second time to super-enlarge it for better details. You might recognize some of the other buckles, and now you'll know they are from horsegear/horseharness/saddlery, also called "horse tack."
