I asked Dr Chua. The blood supply could have been cut off, resulting in the blackened skin discoloration. Dr Chua asked. There was also yellow pus seen on the lower side of the lump. Nature sometimes present the veterinarian challenging problems to solve.
Rabbits are well known to have hard abscesses.
This looked like an encapsulated abscess commonly found in the internal organ of sheep and caused by one of the deadliest bacteria in the world. The bacteria is said to be present in the soil and causes Meloidosis in sheep and as well as killing people. This bacteria forms cheesy or thick pus-like abscess in the internal organs of sheep and is well documented in sheep medicine. The owner wanted to euthanase the rabbit. Maybe, just this one.