Liz Buchanan BVSc MRCVS
Hello! Like all lumps, this will be impossible to definitively diagnose by eye. Biopsy may reveal something benign e.g. a lipoma or cyst, or something more sinister, such as skin cancer. Most vets think about the rough probabilities in either direction, about the state of the patient as a whole, about the ease of surgery at that site. They will then decide with you whether to go from here; whether to 'wait and see,' to fine-needle-aspirate, to biopsy, or to attempt to remove the lesion. All of these options have pros and cons and some techniques are better employed in some situations than others.
Wishing you all the best in finding the best 'next option' for you and the owner of this bum!