In the comic Invincible (a super hero comic series) we meet Monster Girl who appears as a 12 year old. But in reality she should be 29. She was cursed by the grandmother of a boy she liked (whom didnt like her). Now she can turn into a monster however each time the monster is bigger and stronger but she becomes slightly younger when she turns back. So become a power monster but get younger. with no cure.
start of the series she is a pre teen and a possible future showed she was a baby that can turn into a skyscraper size monster.
anyway next year amazon prime will adapt the comic into an animated series with monster girl being cast so will appear.
Trailer briefly does show her turning into a monster at the endhttps://www.youtube.com/watch?v=JuMiq7eg91M