Lots of horses have whorls down the underside of their necks. Most of the whorls are small enough that they don’t make a lot of difference. A small whorl will be a small spot in the neck where the neck will be straighter. There will be a little bit of extra muscling there.
As the whorls get bigger the effect they have will grow. Long whorls the length of the neck will make a major difference in how the horse carries the neck. What that difference will be depends on the type of whorl. There are two basic types of whorls that take up space under the neck.
A zipper whorl is narrow and tight. The hair grows towards center making a ridge along the middle of the whorl. These whorls lead to an arched neck. A high arched neck if they are only on the neck, or a low neck that is carried forward instead of up but rounds nicely if the whorl goes down into the chest. These horses have a natural tendency to carry themselves balanced and working off the hindquarters.
An open splaying whorl, for lack of a better name, has hair that grows outward from center. If there is a ridge of hair it is along the outer edges of the whorl, circling it like a frame. The neck will curve downward into these whorls. Often horses with open splaying whorls will be built down hill and want to carry their noses in the air. With poor riding these whorls can accompany horses who will be uncomfortable and poorly balanced. They need help finding a good way to carry themselves.