A hard lump on a dog's neck may be caused by recent injections, lymph nodes, salivary glands, or parasites. It is best to take the dog to the pet hospital for examination to confirm the cause and treat it promptly to avoid delaying the condition. If it is drug residue caused by recent injections, you can apply hot compresses to the dog several times; if it is parasites, swollen lymph nodes, or salivary gland cysts, surgical treatment is required.