What could this be?

The dog is 10 years old, has allergies and arthritis. This started out small so we thought nothing of it, but it’s bigger now and hasn’t gone away. What could it be?

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!

Here is a bigger and better article about lumps that I wrote some years ago:  https://vethelpdirect.com/vetblog/2019/11/04/my-pet-has-a-lump-what-should-i-do/