And sometimes it is the protagonist who becomes the monster. Amazon includes an excerpt of Transformed and Milked:
“As soon as she began to speak, Abby felt the warmth shifting… Her figure was now curvaceous to an extreme, but that wasn’t all. A pair of tiny, nublike horns grew from her head, small but still obvious – and she recognized them as a cow’s horns, the kind that the females of some breeds had. Even if she hadn’t recognized them, the skinny, hair-tufted tail that now extended from the base of her spine, just above the waist of her skirt, would have made it obvious.
Somehow, the delicious cream had turned her into some kind of human-cow hybrid.”