The Ring of Transformation was a quite powerful artifact. Its user wields the power to transform anyone or anything. One could use it to change a person's body in just about anyway possible except for age and size. They could change people into animals, vice versa, or anywhere in-between. They could turn them into inanimate objects or animated objects. They could even bring ordinary objects to life by changing them into people or animals.
While a Ring of Transformation was as powerful as a Ring of Age, there were some things it couldn't do. It could not alter age as that was in the Ring of Age's demesne. And asides from turning people/animals into bigger or smaller animals/objects, it couldn't really alter size either. And unlike the Ring of Age, it could not alter reality. The best it could do was alter personalities, but they would still remember what they were before. And as such, they couldn't really change the world to fit the changes made, though altering clothing was little trouble.
Morgan le Fay had considered giving Mary Elise her own Ring of Age, but she thought better of it. Merlin's girls would be far more experienced with their Rings than Mary would be, so the best solution was to give her something they wouldn't expect. Besides, it would be more entertaining this way. Although the ring was a gift from Morgan, it was neither inherently good nor evil in nature. It was simply a tool to be used so it couldn't corrupt Mary Elise in itself. But Morgan was sure the lust for power it will awaken will.
She watched eagerly as Mary Elise headed home. She didn't know what she carried yet, but it'll reveal itself soon. The Ring of Transformation didn't carry the same kind of awareness or auto-correction as the Ring of Age, but it will want to be used. In all likelihood, Mary will probably first use the ring by accident and Morgan wanted to see who would be the first victim.