Those mustangs running in the upholstery are part of the deluxe interior package, or Pony packge. They are reproduced. TMI seems to have the best quality reroductions. They come in varios color combinations based on the rest of the interior.
Beach Pony is correct.It's called a Pony Interior and from what I hear it' supposed to add 10% to the value of the car if it comes with it??? And there are quite a few color combinations.I have the Blue & White: http://www.putfile.com/pic.php?img=6881236
It was a '65-'66 only option. The deluxe interiors from '67 and on did not have the running horses on the seats, athough the aftermarket has made upholstery that included the running horses to fit other years.
I have a 66 coupe that had the optional deluxe interior in it, and along with the walnut grain trim on the glove compartment lid, trim on the console and the wood grain on the instrument bezel, and the interior door handles, it also had moulded door panels and the well known "pony" interior appointments.
I have found that while TMI does have and offers the very best quality interior replacement for my pony seat covers, in my case, the "running pony" panels were not the correct color, and TMI admitted this. I had to purchase the correct color panels and have my upholstery shop take the panels out of the new seat covers and carefully hand stitch in the ones with the correct color.