This rig was used in a production test for several things. The main purpose was to test different quadruped rig designs to see what worked best for the animator.

The leg setup used a double knee joint to help mimic the knee cap of a horse. This just helped in skinning the character.

A control interface using the PEN Attribute Holder was created to remove some of the controls from the screen and solve a clutter problem.

The spinal column and ribs are what make this rig work so well. It uses spline IK with a double spine so that I could isolate twist rotations from the other two axis giving a true twist down the spine.

The ribs had just a single control for this test as we didn't need much more. It allowed for the belly of the character to be animated in a wide range of motions.