6. The Thing
What Its Deal Is: Not so far removed from the Slither Parasite, the Thing is another parasitic life-form that exists to absorb and assimilate any native creatures from the planet it finds itself on. Obviously, that planet is Earth and soon into The Thing, we can see exactly 'assimilate' means in practice. With the ability to split itself into multiple parts (like a cell) and the ability to copy the form of those it absorbs, The Thing really would be an insidious and unstoppable nightmare to have to deal with and one which would probably get the best of us in the end.
Why Itd Be Unstoppable: The Thing shows a modicum of intelligence, thanks to gaining sentience from its human victims, and hides in plain sight as those it has already assimilated, while plotting its next course of action. A terrifying prospect, and couple that with the fact that a single cell can act of its own accord while simultaneously still being a part of the whole wed really have no way to halt The Thing's onslaught. You could be sitting right next to The Thing right as you read this. It could be your girlfriend, your sister, your dad it could be in the mug youre drinking out of right now. If The Thing did exist in reality, we wouldnt. At least not for long anyway.