I'm brand new and wanted to purchase a Yorkshire Terrier as my first dog, and was surprised to see they don't exist in the game? I was wondering why?

A load of breeds which had no active breeders were removed from the game a while ago. 

Here's info on how to get a breed added back: https://horsephenomena.com/viewpost.php?id=259776

List of existing Yorkies: https://tinyurl.com/3f4edyt3