This story from the New York Times tells the tale of Chaser, a Border Collie from Spartanburg, SC, with a vocabulary of 1,022 proper nouns. This tops the old record of over 200, held by Rico, another Border Collie in Germany.
It's worth noting that all dogs aren't this smart. JoJo, for example, needs to be re-taught "sit" almost daily. And some of the remarks about horses are tendentious; I studied mathematics with Dr. Cleverhans and found him to be a very astute teacher.