Well, I've been beaten to it on Sarah Palin and Margaret Thatcher, and while The Thing has the scariest female character ever it's too old to be recognised and she looks too normal. That's why she's scary. She only might be it.
Hmm. When you realise that the catgirl craze is actually the prophesised Cthulhu cult, they're scary, but maybe that's not what you want either.
Someone's mentioned the L4D witch.
How about this mean b____ [http://images.wikia.com/asheron/images/3/34/Olthoi_Queen_%28Royal_Hive%29_Live.jpg]? Might be hard to pull off, especially at the proper scale.
Vampire Willow's far too sexy.
Oh! Hey! Idea! Playing the role would be a pain in the posterior, but here's a scary female from recent popular culture [http://observatory.designobserver.com/media/images/WEEPING-ANGEL.jpg] you may be able to imitate well enough. Another view here [http://i.telegraph.co.uk/multimedia/archive/01627/weeping_angel_1627630c.jpg]. Don't forget the mouth details [http://www.cosplayisland.co.uk/files/costumes/625/48680/Weeping_Angel.jpg].