In a book I read, there's a scene where a soldier is making it with a fantasy woman - he can't tell whether she exists, whether it's all a dream, what - and as she's riding him, she turns into this creature called the Shrike - a metallic, barb-wire and scythe-wrapped thing - that has jaws in in its crotch.
And, sinking his flesh into the cutting flesh of the thing, he throws it off him, just as the jaws are snapping shut
This was long before Teeth, of course.