If you're a dog owner, you will be no stranger to lumps and bumps on your fur baby, which sometimes appear overnight. Finding a lump on your dog can be scary, but if it's the common sebaceous cyst they are generally not harmful. Sebaceous cysts in dogs are small pockets that form in and under the skin. They can vary greatly in size, and appear almost anywhere on the dog's body. The good news is they are benign and usually harmless (unless they get infected)