Dear wofahulicodoc: Remember the wide gulf between English and Western riding styles
I believe "crupper" is and English term. I don't ever remember seeing one on a Western
saddle. And if there were one, the cowboys would ;have called it something else.

Incidentally, there is a related term - "come a cropper" meaning to fall off your horse;
or as a metaphor to have some notable misadventure.