At least one of the flock of sheep kept at the Tierra del Fuego research station was infected by another, unidentified Thing assimilant. Subsequently, the crazed and paranoid R.J. MacReady slaughtered most of the flock upon his arrival, correctly believing that one of their number was infected, though failed to actually kill the infected sheep. After MacReady was tranquillised and taken away, station commander Agapito Quitana ordered the cook, Pablo to butcher whatever meat was still usable from the dead sheep. As he was doing so, the infected sheep approached him, and Pablo went to pet it, only for its wool to suddenly engulf and begin fusing with Pablo's right arm.
Pablo tried to free himself by shooting at the Sheep-Thing - which rapidly grew in size to a mixture of vaguely sheep, humanoid and serpent-like features, likely absorbing the other sheep in the process - but was unable to do so, though his efforts at least attracted the attention of Agapito and the other soldiers, who watched helplessly as Pablo's body was fully absorbed into the Sheep-Thing, leaving only his head and left arm visible. Agapito ordered his men to shoot at the Sheep-Thing's head, which appeared to disable it, despite Agapito's second-in-command Ramon warning him that this would not be an effective tactic. Sure enough, a tentacle emerged from the mouth of what remained of Pablo, and ensnared a soldier who got too close, before Pablo's head distended itself out of shape to devour the soldier.
After this, the Sheep-Thing split into two smaller Things: one with a long, serpent-like body, and a large, blob-like Thing covered in tentacles that ended in mouths filled with razor-sharp teeth. Finally heeding Ramon's warnings that fire was the only effective weapon, the soldiers covered the two Things with gasoline, which Agapito ignited with a flamethrower, killing the Things. Before it died however, a human mouth emerged from one of the Things and called for help in Pablo's voice, unnerving his former colleagues.