I don't know about the Mustang's legs. Maybe so.I didn't know a lot of that stuff, Lepp! I picked up some info from a book Margeurite Henry wrote, but not everything you just put down. Like the extra rib, or the difference between a Medicine Hat or war bonnet.
One interesting thing that was said in that book, was that "true" Mustangs have their front legs attached differently; their legs make more of a "v", where blooded horses' legs are supposed to make more of a square. Any idea if there's truth to that? I haven't spent enough time around blooded horses to see for myself. Mustangs have been about all I've had. (they're my favorite) And when I am around a "pure" horse, it never crosses my mind to see how their forelegs are shaped!
If you google Medicine Hat horses, you'll see a LOT of photos of horses that people claim are Medicine Hats. But now that you know the difference between a medicine hat and a war bonnet (and the bit about the shield), you'll be able to spot the real ones.