There a number of different things that can cause this kind of a lump on a dog. It could be an enlarged lymph node (or gland). This can occur in response to inflammation or infection in that area of the body (like a tooth abscess), or lymph nodes can get large from cancer. The lump could be a benign or cancerous tumor. Another possibility is that it is an enlargement associated with a disease of the salivary glands.
Diseases of the salivary glands include:
Salivary mucoceles, or sialoceles, result from damage to the duct or gland, with subsequent leakage of saliva into the tissues. The sublingual (under the tongue) and mandibular (under the lower jaw) salivary glands are most commonly involved. The sites for mucoceles include cervical (neck) mucoceles, sublingual mucoceles (ranulas), and less commonly, mucoceles of the pharyngeal (throat) or orbital (eye) region.
Fistulas---Salivary gland fistulas occur infrequently in small animals, and they are usually the result of trauma to the parotid salivary gland or duct.
Sialoadenitis---This is an inflammatory reqction in the salivary glands. This occurs infrequently in small animals. The zygomatic salivary gland (the one underneath the cheek bone near the eye) is the most common gland to be involved.
Neoplasia--Tumors of the salivary glands are rare. The parotid and submandibular salivary glands are the most susceptible to tumors.
If the lump in your dog's neck (and is not a salivary gland itself) then it is most likely a mucocele. These are soft, fluctuant and nonpainful.
With sialoadenitis, the signs are specific to the salivary gland that is involved.
The zygomatic salivary gland produces signs of a protruding eyeball, tearing, strabismus (where the irises look like they are looking off to the side), reluctance to eat, extreme pain on opening the nouth, inflammation of the oral mucosa near the papilla (the opening of the zygomatic salivary duct), and pus-like discharge from the duct.
Parotid sialoadenitis is a painful, warm, firm parotid salivary gland with pus-like dishcarge from the opening of the duct. The parotid gland is located just in front of the ear on the side of the dog's face.
Most dogs with salivary gland tumors are presented because of a firm mass in the region of a salivary gland.
The diagnosis of which disease the dog has is based on history, clinical signs, radiographs, and histopathology (biopsy). The diagnosis of mucoceles is based on palpation, aspiration of a clear or blood-tinged , mucinous fluid that is consistent with saliva. Sialoadenitis is based on the very painful signs, a high white blood cell count (in the blood of the patient), and on histopathology.
The treatment for mucocele is usually excision and removal of the pocket of saliva and the salivary gland from which it is originating.
For Sialoadenitis, the treatment is to drain the zygomatic salivary gland by surgically opening the plugged duct where it opens in the mouth. This is doenn under anesthesia. The antibiotics used must be based on a bacterial culture and sensitivity of the fluid taken out during surgical drainage.
If it is an enlarged lymph node due to infection, then antibiotics wouldl be the treatment, and surgical removal or drainage of the offending infection would need to be done.
If it is a tumor...then surgical excision would be the next step.
At this point, the next step is to take him to the vet so that he can be examined. The vet will likely put a needle into this lump to do a test called a fine needle aspirate. If the lump is full of pus or saliva, this will greatly help with the diagnosis right away. If it is more of a solid lump, then some small amounts of material will be aspirated out and smeared on a slide to be sent into the laboratory for testing. The pathologist at the lab will be able to tell what this lump is, about 50% of the time from this type of test.
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.