[90-00-09]

My understanding is that such robots do exist and have a different application. For example, balancing on longer hind legs by lifting the rest of the body up, then making a jump over a gap is likely easier than when using wheels.

In terms of landing on the back: if I remember correctly, one of these prototypes--from Boston Dynamics?--is able to rotate its legs 180 degrees to push off the ground when landing on the back.