Yes, I’ve heard of that particular mustang, but… she is merely one. You would have to have a much larger percentage of mustangs being successful if you wanted people to accept them as a premier endurance horse. One just doesn’t cut it.
And while an “Indian shuffle” might be fine for racking up miles for someone who is happy to come in last all the time and just wants to do the sport in a casual way, it sure as heck won’t get a horse down the trail at the speed that the real competitors desire. Again, I don’t want to burst your bubble, but…the Arabian horse has consistantly shown that its breed is the premier one suited for the sport at both the highest, and lowest, levels, with a competitive desire to be the best it can be.
And I don’t even have an Arab. I’m just smart enough to recognize how the top competitors get where they get.
If you just want to mosey down the trail, I’m sure a mustang would be fine, just as almost every other breed on the planet would be. If you want to win, or be competitive, you need a horse with the same level of committment, and the body/genetics to make it a reality.