I had one many years ago, and it did cause aggressive stallion like behaviour including grunting, teasing and mounting. Also she became aggressive towards her handlers. This was a long time ago and due to her age we decided not to have it removed and to have her pts. Treatment nowadays is a lot more advanced and far less invasive. Also symptoms vary tremendously from mare to mare. Have they ultrasounded her ovaries??
17 years ago my daughter's pony was diagnosed with a tumour on her ovary. She was presenting with behaviour like a stallion. The vet knew immediately knew what was wrong. A blood test processed at the vets confirmed the problem. He removed the ovary and tumour during an overnight stay at his surgery. The mare made a full recovery and after a few weeks, once the testosterone that had been produced by the tumour worked out of her system. She returned to being the sweet pony she had been before. My 13 year old daughter continued ride her until she became too big for her and the pony then gave years of pleasure to other children. Hope this helps, good luck and best wishes.