I present to you a rare species of monster girl: an Alp! Here's an excerpt from the Monster Girl Encyclopedia:
"Alps originate as incubi, i.e. men who undergo a mutation into female monsters. Under no ordinary circumstances does a human man turn into a monster.
(...) Alps come about when an incubus wishes deep in his heart to become a woman, or to be joined with a man. The monster mana that made him an incubus is normally used to improve his production of essence, but in this case it destroys this very function. This strips him of his ability to produce essence and empties him of it. Monster mana pervades his body, and he soon finds that he is a monster."
Basically, this monster is a human man turning into a tomboyish Succubus. Don't get it twisted, though, as Alps eventually become just as lust-driven and lascivious as your average Succubus.
Enjoy!~