1,022 words - largest vocabulary of any known dog

Chaser, a border collie who lives in Spartanburg, S.C., knows 1,022 proper nouns, a record that displays unexpected depths of the canine mind. John W. Pilley, Chaser's owner, bought her as a puppy in 2004 from a local border collie breeder and started to train her for four to five hours a day. He would show her an object, say its name up to 40 times, then hide it and ask her to find it, while repeating the name all the time. She was taught one or two new names a day, with monthly revisions and reinforcement for any names she had forgotten. It was hard to remember all the names Chase had to learn, so he wrote the name on each toy with indelible marker. In three years, Chaser's vocabulary included 800 cloth animals, 116 balls, 26 Frisbees and a medley of plastic items. Source