Which of the following is considered an unsoundness of the head?

Parrot Mouth is considered an unsoundness of the head because it refers to a specific dental condition in horses where the upper jaw extends beyond the lower jaw, leading to misalignment of the teeth. This can affect the horse's ability to chew food properly and can lead to other health issues if not addressed. The alignment of the jaw is crucial for effective grazing and overall nutrition, making Parrot Mouth a significant concern in horse evaluation.

The other options relate to unsoundness in different parts of the horse's body. Navicular Disease pertains to a specific issue in the hoof, Bow-legged describes a conformation fault in the limbs, and Capped Knee refers to a swelling at the knee joint. While all these conditions can impact the horse's performance and health, they do not involve the head or mouth, which is the focus of the question regarding the unsoundness of the head.