He's such a cutie. I honestly don't know what it is, but the color makes me think it's not a tumor. Dark brown or black skin looks like dead tissue to me. I guess it could also be melanoma, which I assume might show up with a dark color. My best guess from the photo is there was some damage to the testicle (perhaps it was tied off with string or a rubber band--this is a common way to neuter some livestock, so maybe someone thought they could do it to him, also done to dock tails and ears in dogs) and the organ lost blood flow for a period of time. This caused the tissue there to die and become necrotic. Necrotic tissue is brown or black, but can also lead to infection very quickly. I am surprised the vet at AC&C didn't give you any more help or information, as it appears he may need more than just a simple neuter. I hope the private practice vet can get you some answers. He's too cute and doesn't deserve this! Also, you don't deserve to get told your bunny has a tumor without any real explanation of why they think it's a tumor and what to do for him. Good luck!
