Originally Posted by tinyliny
It looks as if some of those horse have draft blood in them, specifically Clydesdale.
In 1540, Indian hostility forced the Spaniards to abandon Buenos Aires and release 12 to 45 horses (warm blood horses descendants of the berberiscan horse of north africa). When Buenos Aires was resettled in 1580, it is estimated that the feral horse population numbered around 12,000. Since they largely reproduced in the wild, the criollo developed into an extremely hardy horse capable to survive the extreme heat and cold, subsist with little water, and live off the dry grasses of the area. Settlers later came and started capturing horses for riding and for use as pack animals. The Native Americans had already been doing that many years before.
Throughout the 19th century a large proportion of the horses were crossed with imported European Thoroughbred, coach and draft horse stallions, and a larger, coarser, long striding multi-purpose, saddle cart horse resulted.
(This information was taken from wikipedia)