Another possibility is that the lump is a lymph node. Lymph nodes are small, bean-shaped organs that are part of the immune system. They can swell up when the dog is fighting an infection or has an injury. If the lump is a lymph node, it will usually go down in size once the infection or injury has healed.
If you are concerned about the lump on your dog's neck, it is always best to have it checked by a veterinarian to rule out any serious medical conditions.