My pony had a melanoma on her chest about 8 years ago. She does have a few now as well, though they have not changed in roughly 8 years so vet is not concerned. The largest one (8 years ago), was found to be cancerous after a biopsy. She would have had to be put down, but surgery was decided. Thank god that was possible because she's my heart horse. I was able to watch the entire surgery with my mum, can't remember how long it took, but she was drugged (obviously) and they did the surgery standing up. After the melanoma came out it was about the size of two fists, or a fist made by a larger man. After the surgery she was stitched up and on stall rest for 12 weeks I believe, and I think it was at 8 weeks I was able to hand walk her slowly.
Today she is fat and happy and in regular work, cancer free. =)