Sandra Bullock's chihuahuas are Poppy and Ruby. We don't often get to see Ruby walking in her upright manner, as Sandra tends usually to carry her. Ruby's the special needs chihuahua born with only hind legs (and tiny flippers in front).
I suspect Sandra carries her more than she does Poppy because dogs that walk upright risk rheumatism or hip problems later in life, and it's a more tiring way for a dog to walk. Nevertheless, Ruby really can walk quite well. "She's like a little dinosaur," says Sandra, "A velociraptor."
Poppy, whom Sandra had earlier adopted from the same rescue shelter, is similarly a special-needs chi having only three legs. Poppy lost her fourth leg in a car crash, but gets around as though not even noticing one leg is gone.