The old Spanish lines were more of a "draft" breed because they originated from the forest breeds. They were favored due to the fact that they were "easy keepers" and could live on less forage then your desert breeds. Remember the old Spanish breeds were war horses. The ancient method of "training" war horses were to bleed them, starve them and then run them though a series of rough terrain. If that didn't kill them then the trained them, fed them well and sent them off to war. The draft shape also comes from the fact that a stockier build meant improved chance of starving through the winter (think surface area ratio). There is always the possibility that somewhere in his past, someones draft horses were released into the herd and since they are well suited to survival, were passed along. Also remember that the old bloodlines were genetically more diverse than the modern breeds (case in point, icelandics have the most genetic diversity of any breed even though they have been isolated since the 900's. That diversity combined with wild selection would produce a fine horse. Mustangs are in essence mixed breed horses with wild selection refining them more so than other breeds. Human breeders tend to breed for horses that look the same. Wild selection breeds for animals that survive similar situations so there tends to be more variability in looks but less variability of function. (example a horse with bigger hooves might be more sure footed than a smaller hooved horse but not as fast as a smaller footed horse. The bigger hooves would survive because they fall down less, the smaller hooved horse would survive because they turned faster, both resulted in successful escape from the predator therefore that variability would remain constant but on the other hand a tall lanky horse would not survive the harsh winters and so over the years, only the short, draftier horses would make it to breed. The end result is that they typical mustang is sure footed, easy to keep (not needing a lot of food), good solid hooves, very aware of their environment (read sometimes skittish) and extremely loyal once you "prove" to them that you are worthy of that loyalty. Of course each animal is different but in general.