Anyway, I wonder if I am the only one who gets creeped out when a cute girl or pretty woman turns out to be a (not nice looking) male monster/alien/creature. This is only when the original form is an unattractive creature, and it takes a form of an attractive human (or appearing human) woman as a disguise for some necessary reason (to trick the hero(es), most likely), though. I don't really mind it much the other way around, when an attractive woman can, for some reason, transform into a monster/non-attractive creature.
And somehow, I don't remember 'monster as a girl' in genre other than sentai. (ETA: Sorry, actually there is that lady Bogal whom I'd completely forgotten (probably because she never did creep me out) from Mebius, which is not sentai, as far as I know. Thanks to jeff_morris for reminding me of her.) Girl as monster, however, I think I've seen before in other genres.