The image below will show a few of the muscles (not all) in the horse's neck. The brachiocephalicus of the Icelandic Horse
is obvious in the horse's neck conformation.
How a horse uses his body and how he is ridden will play a part in the development of his neck muscles. In the first group of images below, the brachiocephalicus muscle shows hypertrophy (arrows). This is caused from the horse's hollow frame and bracing against the bit. The underside of the neck gets large and the muscles along the top side of the neck are under-developed.
The same necks will be found with Big Lick Tennessee Walking Horses. The underside of the Big Lick horse is always more developed because the neck and back are held in a hollow position.
The hollow frame with the neck pulled up breaks the horse in half and causes an inability of the hindquarters to perform their job of powering the horse.
The second set of images shows horses using different muscles via the "neck telescoping guesture". The NTG is a component of collection. The complexus muscle is developed with the NTG; the scalenus muscles contract which raises the root of the neck. It opens the throatlatch which opens the horse's windpipe and leaves more room for the back of the tongue.
It allows the horse to work "through" his body; the contraction unifying the neck and the back.