The Thing

You're stationed at an isolated outpost in Antarctica when a helicopter arrives intent on shooting a dog it had been pursuing across the tundra. Things get out of hand and the occupants of the chopper have their ability to pursue curtailed, more permanently than they'd like. Not too long after this carnage it becomes apparent that the pursuing shooters had a pretty good reason for wanting to remove the dog from the land of the living as the dog turns out to have the same aspirations as its pursuers, albeit in various guises, transformations, metamorphoses and reconfigurations.

While this was made in 1981 it remains to this day one of the greatest and most engaging pieces of horror movie making that has ever been created.