Chaser, a border collie in South Carolina, was trained over several years by a psychologist who worked with her for hours daily. Published testing showed she could reliably retrieve items from a set of 1,022 named objects, and that she distinguished the names as labels for individual things rather than as commands to fetch.
The more interesting result was inferential. Presented with a group of familiar toys plus one unfamiliar object, and asked for a name she had never heard, Chaser selected the new item. This is learning by exclusion, a strategy also seen in young children, though in Chaser's case it does not demonstrate that she understood the word as referring to a category.
The finding is often reported as evidence that border collies are the most intelligent breed. It supports no such ranking. Chaser's performance reflects an extraordinary quantity of one-to-one training; comparable feats have been shown in a Border Collie named Rico and in individual dogs of other breeds. What the work established is a ceiling, not a breed average.