Large lump near tail

[sk8mon]

I need help in identifying what this large, dark bump is near the tail, what might be the cause, and how to treat. Thanks.

 

[calioutlaw1a]

It could be a variety of things: Inflammation and cellular proliferation in the area in response to trauma, lymphocystis (normally more white in appearance), cancer, and many more.

Is the bump smooth or bumpy? Did it suddenly appear recently, or has it been growing over time. Does it seem to be getting larger or staying the same size? How long has it been there?