My Shi-itzu Buggz who was born without the use of his back legs taught himself how to hop with his back legs and after a while he gained enough strength in them he can walk like a little old man.
We would take him everywhere with us, hiking, swimming, any activities we did became therapy for him to strengthen his back end. His favorite thing to do is swim, the first time he swam he jumped in one of our watering holes that had stairs leading into it, you basically jump into water 4 ft deep, he had no problem! We couldn’t keep him out after that! The neatest thing is he doesn’t even use his back legs, he tucks them up under his belly and literally does the dog paddle!