[jpwn]

Are you setting it in modern times? Or just doing a modern retelling of the story?

If it's in modern times, I think it'd be cool if the witch ran a petting zoo or rented the victim for kids parties or something like that. Maybe the victim works a graveyard shift so she sleeps during the day. That could tie into how she starts to figure out what's happening to her. Maybe her niece/nephew has a birthday party and the victim unwittingly misses it as a human but appears as a horse. Of course, that's just my idea. It's your story so feel free to take it or leave it.

With the hooves, it's usually done one of two ways - either the hands curl into fists which become the hooves, or the middle finger/toe enlarges into a hoof while the other fingers/toes whither away. The latter would probably be the best way for the feet to change, but again that's just my opinion.

Another she could start to suspect something's up (assuming a modern setting) could be torn clothing, especially if her panties suddenly have holes where her tail grew in. Maybe her vegetarian diet as a horse starts to affect her diet as a human.

Hope that helps!