I've got an odd source that has some bearing on your question. "Monsters of Patagonia" is a book by Austin Whittall devoted to cryptozoological reports from Patagonia. Many of the cases he discusses are tied to either native folklore (tribal memory of now extinct 'megafauna') or the reports of early explorers. Toward the end of the book, Whittall discusses historical reports of canids (dogs, wolves, etc.) from Patagonia. WITHIN this section (p.254-255), he discusses the native Patagonian dogs. The breeds possessed by the native Patagonians have either now gone extinct, or have been so intermingled with other breeds that they are lost. Thus, we cannot easily get at the question of where the Patagonians got their dogs (brought over the Bering landbridge, domesticated an American wolf independently, etc.). The Aonikenks of southern Patagonia hunted with a large dog that resembled a deer hound. The Spanish explorer Sarmiento de Gamboa saw them in 1579, and described them as having a brindled coat and were larger than Irish wolfhounds (I'm pulling from Whittall's text here, p. 254). Direct quote from Sarmiento de Gamboa, "It was noteworthy that our dogs, and those of the natives, flew at each other until they came within four paces, when they turned round without touching and we could never get them to attack again." The Selk'nam of Terra del Fuego had not a domesticated dog, but a domesticated fox (now extinct). Whittall does not have any recorded Spanish thoughts on the Fuegian 'dog', but I would not be surprised if there were such accounts out there. Whittall cites an old book "The Natural History of Dogs: Canidae or Genus Canis" by Hamilton Smith (1840)m which has sections on the Patagonian dog and the Fuegian Dog. And, it is available free on Googlebooks: http://books.google.com/books?id=jyYOAAAAQAAJ&printsec=frontcover&dq=the+natural+history+of+dogs&hl=en&sa=X&ei=0V_TUsGQJ6fl2QWr24CoBQ&ved=0CC8Q6AEwAA#v=onepage&q=the%20natural%20history%20of%20dogs&f=false