A pimple on a dog’s back may be caused by tumors, tissue hyperplasia, insect bites, or allergies. If the pimple grows larger, it may be a tumor. If the pimple is pressed, the dog will feel pain. If you feel it, then it may be tissue hyperplasia. It is best to take the dog to the pet hospital for examination to confirm the cause and then carry out targeted treatment to avoid delaying the condition.