There are many possible diagrams of the anatomy of horse tissues. This is because there are many layers of muscles.

Superficial- More external or towards the surface of the body.

In this picture it shows the muscles that are closest to the surface of the skin, making them superficial.

Deep- More internal, or towards the center of the horse (These are ones that you wouldn't be able to see their outline or feel on the horse)

Intermediate- In the middle.

A lot of the muscles seen in this picture form the bulky part of the horse anatomy that you see just beneath the skin, that you can feel. Although they may be more developed in some horses than others, you'd still recognize them as the outline of the horse in pictures or diagrams where the whole is seen.