Where protagonist were humans but became monsters, are not human from the start or became human but retain their monster side. This list has everything i can find regarding the theme like medieval monsters, demons, vampires...
[Organized in alphabetical order]
Princesses finding love, growing into strong rulers, escaping their palace walls, facing arranged marriages, taking back their thrones, or trapped by a curse.
Mostly shoujo manga were at least one of two lovers is nonhuman, taking the form of a dark creature. It's the typical storyline: The female lead becomes the lover of some creature by a curse, a contract, forceful kidnapping...