Let me see if I can come up with a few suggestions for you to consider.
Devil in a blue dress
No one would ever suspect the girl in the micro mini blue dress, pearls and shades (slipped down allowing her eyes to be seen) was really a devil girl. Nope not even with her hooves, tail, horns, wings, fangs razor sharp nails, and red skin clearly visible. It's a perfect disguise.
Invasion of the Bee Girls
Nerdy scientific girl transformed into sexy queen bee - complete with abdomen/stinger and wings. Can be the result of her potion or a sexy variation on the classic Fly movie franchise.
variation on the classic werewolf theme as his older date is revealed to be a real catwoman - cougar / leopard / cheetah / panther under the romantic full moon.