Ya'll were talking about the horse in Hidalgo and never having seen a mustang with that color. Well apparently there are plenty. Check out this site I found while searching on Google for Sabino pattern descriptions:
http://www.mustangs4us.com/Horse%20Colors/sabino.htm
Page down about 5 times and look at the horse on the right hand side. He looks just like Hidalgo. Same color and everything although kinda scrawney.
These are the ones that I really think have some appaloosa in their background. The roaning type is an appaloosa coat pattern and I've seen foals off a quarter horse mare and an appaloosa stud that had that really ragged outline spots.