My dog keeps scratching a lump behind his ear. What could it be?

Thank you for posting the picture of the lump on Leo. There are several possibilities of what it could be but I suspect it is one of 3 things. A sebaceous adenoma which is a benign growth of the oil gland, a cyst, or possibly a tumor called a mast cell tumor. A veterinarian will take a needle aspirate and look at the cells under the microscope to assess if this is something that is concerning or not. If it does look like a mast cell tumor then surgical removal will likely be recommended. Best of luck and I hope it is nothing concerning. Thank you for using Pet Coach.