Boo Boo was the world's smallest dog from 2007 to 2013. She stood at a height of according to the Guinness Book of World Records.
Boo Boo was replaced by Milly, who stood at , who was another Chihuahua, as the World's Smallest Dog. Other contenders for the record included Beyoncé, a Dachshund-Chihuahua mix, and Scooter, a Maltese.