My hamster’s leg is red, swollen, and large; after it bit itself the area bled a lot and the surrounding flesh turned black
Expert Answer
From the photo, the hamster has a skin infection, and a ruptured tumor causing infection or necrosis cannot be ruled out. The symptoms are already serious, so take it to a vet promptly for a detailed check and effective treatment. If the hamster has a tumor, treatments include tumor removal, chemotherapy, targeted therapy, immunotherapy, or herbal medicine.
Black flesh after a self-bite that bled a lot is tissue dying, necrosis, and that's an emergency for a small pet, I'd get to an exotics vet today, not wait. Keep him clean, warm, and separate from anything that stresses him, but the dead tissue needs a pro to assess, maybe cleaning or removal. Small animals crash fast, so speed matters, and don't try to treat the black part at home, get him seen.