Are the creatures mutants for the fog?

No, the army guys explain that the military was experimenting with a "window" to other dimensions. The creatures had come through it.

Stephen King has a lot of connections throughout his books. This other world is accessible through portals in several of his books, or so seem to be the same place. Referred to as the Wastelands in the Dark Tower, and the Blasted Lands in the Talisman, and both have people "traveling" from our world to this place one way or another. It's implied that this is actually a version of our world in the far distant future. It is is also eluded to in From a Buick 8, where the car is a portal and things come from that dimension to ours. Great book by the way. I'd be really surprised if there aren't many more, but nothing jumps right out at me.