In John Carpenter's 'The Thing' (1982), a group of American research scientists are disturbed at their base camp by a helicopter trying to shoot a dog. The helicopter explodes, leaving no explanation for the chase. During the night, the dog mutates and attacks other dogs in the cage and members of the team that investigate. The team soon realizes that an alien life-form with the ability to take over other bodies is on the loose and they don't know who among them is real anymore.
Carpenter strips the alien down to imitation and revolt: skin opens, faces flower, dogs become limbs. The horror is that any body in the room — including yours — could already be the betrayal.