There are different kinds of "mustangs"
BLM mustangs are feral crossbreeds of draft, thoroughbred, and other ancestry. Some have Spanish ancestry, but most have little to none. They've survived the elements and are hardy and come in all shapes sizes and forms. You can get one for just about anything, drafty, light, or in between.
Kiger mustangs are a strain of BLM horse seperated because they had Spanish traits, domesticated ones are becoming more of a designer breed, growing up to 16hh, still nice, but NOT mustangs, in my opinion. The wild ones are still pretty much unmessed with.
The Sulphur, Cerbat, and some of the Pryor Mountain BLM horses are unchanged spanish horses some of the Sulphur horses can be registered with the Spanish Mustang Registry.
Then, there's the original, I'm prejudice, because I have one of these. 12.5-15hh, can run 100 miles in a day and do it again come morning. Can work all day without being shod, survive on grass where other horses starve and don't need grain. They are renowned for their endurance. And they're just gorgeous.