The border collie Chaser was documented as knowing 1,022 unique words.
Chaser was trained by psychologist John W. Pilley, who used play, repetition, and carefully named toys to build her vocabulary. By the end of the research, she could identify hundreds of objects by name and retrieve them on request.
Her abilities involved more than simple obedience. Chaser could distinguish individual toy names from broader categories, such as toys, balls, or frisbees. She also demonstrated “fast mapping,” inferring the name of an unfamiliar object by excluding names she already knew.
The number is sometimes loosely reported as “more than 1,000 words,” but 1,022 is the specific figure associated with the published testing. Chaser was a dog learning spoken labels for objects, not a human-language speaker; researchers therefore describe her achievement in terms of word recognition and learned associations.