Runty little Brandy (full name Heaven Sent Brandy) weighed a shockingly scant two pounds at age four, thus made it into the Guinness World Records in 2005 as world's smallest know living dog.
Paulette Keller's chihiuahua came from a good line of well-bred chihuahuas, but was the runt of a litter of five. She stopped growing at all by age one.
Good breeders will not include such runts in their ongoing breeding programs because they generally come with serious health problems. Relative to many exaggeratedly undersized chis, Brandy has fortunately remained largely healthy, though she does have a few issues or peculiarities.
The neurological element that makes many (not quite all) chihuahuas shiver as though cold, even when they're not cold, is exaggerated in Brandy. When she got her first rabies shot as law requires, she nearly died from the reaction.
Her mouth and teeth are so small she cannot chew up even the smallest kibble, which has to be crushed for her, and mixed with scrambled egg. And Brandy never developed the ability to bark, because her longs are too tiny to work up the sound.
She became famous thanks to the Guinness record, an appearance on Oprah Winfrey's show, Good Morning America, and Jay Leno's Tonight show, plus a whole lot of international media attention. She's quite the world-travelled chihuahua, and because she can't bark, Paulette is able to take her anywhere in her purse without anyone knowing.
She was literally not permitted to jump off furniture, as there's a good chance she'd've broken one of her twig-thin leg. It's not safe to let children play with her unless they understand she is far more breakable than a normal sized chihuahua. Yet she sleeps with her companion Tyson (the large dog in the photo below) every night without fear.