I know I'l probably get flamed for this but I think Genma is a good
Canidate as the Rapist if she was raped.  I mean he is a bastard.  As a
matter of fact I am presently writing a fic in which Genma rapes Ranma-chan
shortly after Jusenkyo.  Ranma-chan arrives at the Amazon village shaken and
terrified by what has happened and asks to be hidden from his/her father.
THhats all I'm saying for now but I think you getthe premise to its' start.

Essentialy Psichotic Maniac