You watched in awe as she changed, and then smiled. "Naw, our parents aren't effected at all. Well, save for being okay with us mating. You are adopted after all."
Nothing much changed but she grinned "Yeah, it means we can mate whenever we want."
"It does mean that, but when you took your changing treatments, you took them all at once! You are going to change into a huge butted pony!"
"Im so excited! I can be your mare, your own p... poneeeeee."
You watched as her changed raced out from her rump, hips widening until she had a proportionately huge, even for a horse, pony rump, starting to be covered in a light velvety fur. Her face began to elongate, head tilted "Changing... too much! Too fast!"
Her mind started going, but you just stood there, cock getting erect at the sight of the mare before you.
"It's okay, you are just my huge butted always in heat pony. It's all anyone else has ever known you as. You with your magic mare slit."
She whinnied as she finished changing, her mind nothing more than a pony with the desire to fuck you. As you exited the bathroom into your room she followed, and took position in the new stall that seemed to be there.