Quadruped - Leg
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
adjustable tarsus (third joint) control
adjustable knee control
stretchy ik limbs with squash option
standard reverse foot setup
Morgan Loomis's tutorial with a spring ik: