Obie has (well had) a Sarcoid on her butt! Obie is the Fearis family project, just turned 6 and still goofy as all get-out. If you don't know much about sarcoids (skin cancer), there is a useful article at Smartpak (click on the photo).
Our vet did a biopsy, lab confirmed the diagnosis, prescribed and we applied some topical cream (Aldara) that promoted an inflammatory response. A week later sarcoid was history. Get them early is my advice... much easier to deal with!