I do not recognize this... But... the good thing is you have that piece of leather with stitching... see the stitching can tell you a lot.
Ok... lesson in stiches
1... IF stitching has uniform spacing and in line clean... = Machine made... = modern.... every stitch slightly different in spacing... hand made.
2... IF stitching is made from "synthetic" threads... = Modern... WELLL at least modern since it would be post Nylon invention / use.
Stitching can be tested easily for this by fraying a small piece and then taking a lighter to it and notice the way it burns and particularly the smell.
I know this does not answer your question if it is horse related or not ... but it will tell you if its more a modern piece. heh
I was in full solve it mode :P