What's this bleeding lump on my 11-year-old dog's rear end?

Thank you for contacting Petco Pet Education Center, formerly Petcoach with your question about Dexter. I agree, that is concerning that this hard lump is now bleeding, and this lump could be something serious. It could be a hemangiosarcoma (malignant) or hermangioma (benign), which are tumors that develop from blood vessels and look like a blood blister. Given his age, there is a chance it could be another type of malignant tumor, like a mast cell tumor, but it could also be benign. Unfortunately without seeing him in person, I cannot say for certain. Any suspicious lumps should be examined by the vet. The vet can perform a test called a fine needle aspirate on it. To do this, the vet sticks a small needle into the lump to get a sample of its cells, and examines the cells on a slide under a microscope. It's usually painless to the animal, and relatively quick to perform. Once the cause of the lump is diagnosed, the vet can discuss treatment options if needed. Give his vet a call to make him an appointment. I hope this helps!