My dog has a lump on his tail and keeps biting it. What is it?

Thank you for contacting Petco Pet Education Center, formerly Petcoach with your question about Toby and for providing a picture of his lump, it's very helpful. That is quite a big lump, and he will be biting his tail because it's bothering him. This lump could be a cyst, abscess, a benign tumor, or a malignant tumor. Unfortunately I cannot diagnose what it is without seeing him in person. Any suspicious lumps should be examined by the vet. The vet can perform a test called a fine needle aspirate on it. To do this, the vet sticks a small needle into the lump to get a sample of its cells, and examines the cells on a slide under a microscope. It's usually painless to the animal, and relatively quick to perform. Once the cause of the lump is diagnosed, the vet can discuss treatment options if needed. Give his vet a call now to make him an appointment. Until he sees the vet, I would put an Ecollar on him to keep him from biting his tail further. If you don't have one at home already, you can buy one at a pet store. I hope this helps!