I have written several posts on the possibility that the supposedly extinct horses that were native to America, managed somehow to survive into modern time, and maybe even mate and cross with European horses introduced by the Spaniards after they discovered America in 1492.
Pre hispanic horses at Lake Nahuel Huapi
Well, there are several pre-Hispanic horses in rock art depictions; at Nahuel Huapi Lake there are several (see the images above), and they represent a horse riding warriors; they were discovered by Asbjorn Pedersen in 1960. He wrote that he was amazed by these horsemen but was even more surprised when he “later noticed that these paintings could be the first tangible manifestation of an extinct fauna, since they do not represent the common horse (Equus caballus), but the American horse (Equus rectidens)”.
My posts on extant pre-hispanic horses
In one post on these horses, I mentioned that pre-Hispanic horses resembled donkeys.
In another I mentioned the possible survival of supposedly extinct megafaunal horses.
And in another I explored the possibility that Native American horses survived extinction.
 Pedersen, A., (1979). Las pinturas rupestres del parque nacional Nahuel Huapi. Anales de Parques Nacionales XIV (1978): 7-43.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2011 International Year of Forests Copyright 2009-2011 by Austin Whittall ©