Which part of the horse is NOT used to evaluate muscling?

The chest is not typically used as a primary area for evaluating muscling in horses. Assessing muscling usually focuses on areas where muscle mass contributes significantly to performance and overall conformation, such as the thigh and shoulder. The thigh is important because it houses large muscle groups that are crucial for movement and power. The shoulder also plays a vital role, as it helps in determining the horse's ability to move efficiently and effectively.

While the pastern can provide insight into the horse's overall conformation and soundness, it is not a major indicator of muscling compared to the other areas. Therefore, the chest does not serve as a key area for evaluating muscling in horses, making it the correct answer in this context.