Scooter (above) is one of four siblings born without front legs. He skips on his hind legs like a tiny kangaroo. This genetic disorder is alas not a great rarity in chihuahuas, thanks to mismanagement of inbreeding.
Below we can see Scooter's family. His siblings are Carmen, Pablo, and Venus, an entire liter of pups born without front legs.
They were provided with front-wheel carts so they can speed around horizontally like any fully limbed dogs, with less chance of joint problems if they always had to walk upright.
Some chis lacking front limbs attempt to walk about on their nubs, causing ware-injuries that get infections, so by and large it is thought best to give them wheels, to which such dogs readily adapt.
Donna Imhoff (a veterinarian of Port Washington, New York) rescued the litter from certain extermination. She says their special needs makes them a lot of work, "But worth it. I love each and every one of them."