When Donna Imhof's four "scooter dogs" were first sprung on the public, only three of them were shown. Why the fourth (the one named Scooter) appeared a few weeks later I never knew, perhaps Donna had already decided to keep one, as the first wave of publicity was aimed at finding homes for the special-needs cuties. Donna invested so much time in them she fell in love with them all, and kept them.
They arrived as pups, much too small for scooters. Plus there was no standardized dog wheelchair in existence. They were always ad-hoc creations for very few dogs.
This time, with four scooters required, a bit of research went into designing something that could be duplicated for any dog shy a couple of legs.
In the trio of photos in the illustration second from the top we see the three pups as the world first knew them, too little for wheelchairs which in any case did not yet exist.
They are Venus, Carmen and Pablo. Venus had the closest things to rudimentary legs, toothpicky twigs without feet or pads. The others are nubbier.
We see them getting physical therapy (photo in-text, right), like little marrionettes learning to walk on their hind legs.
They tended in any case to walk upright without ecouragement, but they also tried to walk on their nubs, which caused them injury and potential infection.
So the marrionete exercise equipment, on a soft surface, both strengthened their hind parts in preparation for wheelchairs when they got older, and kept them from injuring their nubs.
The very last photo (beneath the videos below) shows Venus, Pablu and Carmen when they first got their wheels.
Puppy pictures of Pablo, Venus, and Carmen as pups in 2007: