The Retuerta horse is a breed that runs wild in Andalucia, in southern Spain. According to this fascinating article the Retuerta horse "is said to be the oldest surviving breed of horse in Europe, if not the world – and the most unique, having no genes whatsoever in common with any other known race on the planet." It continues:
Studies into the Retuertas have been ongoing since the 1980s. The DNA of this endangered species of horse has been closely examined along with that of another ten breeds from Europe and North Africa, and was found to have nothing whatsoever in common with any of them. Neither the English thoroughbred nor the Pura Raza Española; neither Arabs nor British mountain and moorland ponies, and were found to be no relation whatsoever to native Spanish breeds such as the Losino, Mallorquín, Menorquín, Asturcón, the Spanish Trotter nor the Basque ‘pottoka’ breed.
There are some more photos at this link.
Update: here is the original scientific study that led to the news cited above.