RIGGING DOJO & GOOSH PRESENT: Quadruped Friendly Rigging!
A Quadruped ('reverse knee') leg is actually quite anatomically similar to a human or other biped leg; the difference is that the knee is higher up and the animal is standing on what in a human skeleton would be the toes. The 'heel' is raised up and forms a second hinge along with the hinge at the knee.
Features of Quadruped Leg Rig
Morgan Loomis's tutorial with a spring ik: