My precious grey arab gelding had melanoma years ago. He had them under the tail and on his sheath and the vet even found one in his rectum. They aren't a problem unless they ulcerate, or are blocking something major. My gelding at age 19 was diagnosed with a large ulcerative melanoma on his sheath that was starting to affect his flow of urine. Not to go into too much detail, but he had to have major surgery at UC Davis with an incredible vascular surgeon. He lived another 7 years. Complications from Cushings Disease eventually claimed his life, but we had many wonderful years together after his surgery. As others have said, this probably won't kill your girl. Also, there have been some positive results from the use of cimetidine (tagamet) believe it or not. Check into it online. Good luck!