The witch had made her spell carefully. Monica would be mostly mentally unchanged, regardless of what happened. She would remember being human, have a human intelligence, and think the same as a human. Though, as the farmer had observed, over time she would gradually get more horse instincts. Instincts she tried to resist but couldn't.
Those very instincts led to the discovery of her true equine ability. The farmer tried to break her, but she was too smart. She refused to be ridden or used for farmwork. There was only one thing left he could do with her. Breeding.
Monica, now Tempest, knew what was coming to her when the first stud, a stallion named Dyson, was brought over to her. But she couldn't help herself! He just...smelled so good! Tempest willingly and eagerly submitted to Dyson, getting mounted and impregnated. On the first try!
At first, the farmer thought it was a fluke. But Tempest's fertility flew in the face of how difficult horse breeding usually was. She would get pregnant on the first try every single time.
Word spread about this incredibly fertile mare. Suddenly, everybody wanted to have a foal birthed by her. Tempest gradually grew in fame, until something happened that catapulted her fame so much that even people who didn't know about horses knew about her.