For a horse with muscle atrophy, or hollows, behind the shoulder blades and prominent withers, this shoulder bridge pad from Tucker in the answer.

• Based on Tucker's vast saddle fitting experience, they offer several styles of bridge pads to accommodate some of the most common conformation issues that interfere with proper saddle fit.
• Designed to fit under your regular saddle pad, the bridge pads are made with memory foam that fill in the gaps between your horse and saddle which provides a more comfortable fit for your horse. 
• Available in ¾” and 1” thicknesses
• Encased in a soft covering for easy care.