A young female in her teen years (13-19) that portrays an image of innocence but behind closed doors performs nasty sex acts that you would not expect from such a nice young girl. (E.g., Anal sex, Deep throat, Ass to mouth, tea bagging, rim jobs, orgies, DP, etc.)
A monster is similar to a freak. The difference is in the age and how you expect her to act biased on her outside appearance.
-"What's up Joe, how was that date last night with that hot young librarian girl? I bet she was lame."
~"Bro you are not gonna believe it, I thought that she was a geek too but I brought her to dinner and then we went back to my place for a drink and before you know it she was letting me get hot in her ass! Man that girl is a monster!!"