In the above photo by Richard Bradbury for a Guinness World Records event on the White House lawn, Gibson the Great Dane (world's tallest dog) hangs out with longhaired chihuahua Boo Boo (the world's shortest dog), in September of 2007.

Alas, runt chihuahuas generally have serious health problems and Boo Boo died before year's end, not even two years old. But she is immortalized on the world wide web.