I just came across this research article about a female Yorkie named Bailey who had learned the names of 117 different toys, and could do a reasonable job of fetching the correct toy on command (although she had trouble learning the names of toys she had never seen before). The article is fairly easy to understand, but if you can't make it all the way through, be sure to go to the bottom of the article where there is a video of Bailey fetching "Gingerbread Man." It's hilarious and cute at the same time! Enjoy!
PLOS ONE: Vocabulary Learning in a Yorkshire Terrier: Slow Mapping of Spoken Words