Horses are athletes just like people. They get injuries, just like people.They get muscle injuries and when they have muscle injuries, it would feel like a knot in your neck. We complain about it loud enough for other people to hear, but horses don't express their pain loud enough for the common horse owner to hear. Muscle injuries are painful for horses and you might start to see their preformance declining or have a behavior change. A horse can get muscle injuries in many different ways, like the saddle you are using isn't fitted right and is restricting their proper movement. Any lower leg discomfort such as a sore foot, abscess or a compromised joint; the horse is going to do what ever it takes to not put weight on that leg, this can effect all the muscles in the horses body. Dental discomfort can effect the horse's neck muscles, the horse will try to resist and brace itself through the neck. It would start in the neck and you would start seeing the discomfort throughout the horse's body. The bridle and bit can cause problems in the horse's poll. This can cause discomfort because of pressure that the bridle is causing if the bridle is not fitted properly. A thick, heavy bit will make the horse have discomfort in its jaw. A thinner bit is more comfortable than a thick one.
Below are red spots on my horse Indy, showing where the discomfort could start: