Even the oldest known breeds today seem to only go back a few hundred, or perhaps thousand years. This leaves a huge gap of time during which humans had domesticated dogs. Given the wide variety of dog breeds, is it known what kind of dogs existed in prehistoric times? What they looked like, what breed today they most closely resemble, etc.
The paper "Genetic Structure of the Purebred Domestic Dog" identifies a few breeds that appear to have ancient origins. Malamutes and Huskies are considered to be similar to ancient sled dogs with some recent wolf ancestry, which given their wolf-like appearance isn't perhaps surprising. Dog skeletons resembling modern Elkhounds found in Norway have been dated to 5000 to 4000 BC, but the genetic results are mixed (they have some more modern genetics, but also a unique marker from recent wolf ancestry).
The current breed most referenced in these discussions would be the Saluki. These are a sighthound- long legs, long bodies, large heart and lungs- developed to hunt by sight and extremely adept runners.
There are petroglyphs and rock arts in what is now central Iran that shows saluki-like hounds dated to ca. 8000-10,000 BC.
There are also Saluki-like drawings found on pottery found in Susa which was ancient Mesopotamia- these are believed to be 6,000 years old.
Additionally, there are drawings of dogs resembling Saluki and Greyhounds in Egyptian tombs from 2134 BC–1785 BC, and they have even been found mummified along side bodies of the Pharaohs in the Pyramids.