Mass below dogs tail/anus that has suddenly changed in size and appearance

My dog has had this mass about the size of maybe a golfball probably smaller for years that I have been watching with no real change to size or look until here recently. It has massively grown in size and the look has change from looking like normal skin to very nasty looking. I am already 10,000 in debt from my other dogs having serious health issues here recently and can't afford another huge vet bill. So was hoping a vet could give me some idea of what I'm looking at. The mass is hard/firm to the touch and doesn't seem to bother him but he is also suffering from degenerative myelopathy so maybe he just can't feel it?!

Hello - I'm afraid that disappointingly, the appearance of a lump is often unrelated to the type of cell inside it - and it is the type of cell that the lump is made from (and the tissue it is growing from) that determines the type of lump it is.  This is a roundabout explanation as to why vets like to diagnose lumps in person, and often resort to biopsies to look at the cells.  Out of context, this could be an inflamed scrotum or even a clever photo of the tip of the penis, but it could also be a lump eg due to the skin expelling a foreign body (such as a grass seed), or a benign lump cancer-wise that has for some reason become inflamed and sore, or a cancer.  I would reccommend that this is seen because the impact on your dogs' wealfare needs to be considered and addressed.  As a vet, I always appreciate clients who are direct about a problem.  For example,  'I found this on my dog and don't know what it is, but money is tight at the moment and we wondered if it NEEDS to be treated / identified for welfare reasons or what the implications might be of leaving it alone.'  I hope that this is, at least, slightly helpful.