The earliest incident of a thing landing on Earth and attempting to assimilate the local population occurred in 1121 AD. The thing attempted to assimilate the local Norse population and nearly did so.
In 1982, Norwegian scientists in an Norwegian science outpost in Antarctica found a frozen Thing in the ice which they accidentally unleashed with lethal results. The Thing managed to infect and kill most of the crew at the Norwegian outpost before being defeated and driven off, forcing the thing to take the form of a dog to better flee. The dog-thing managed to escape to an American outpost where its Norwegian pursuers were unfortunately killed by the Americans who believed that the Norwegians have snapped.
They placed the dog-thing in a kennel with the rest of the sled dogs while they investigated the Norwegian outpost where they found mutilated non-human bodies, one of which they took back their base for dissection. The dog-thing assimilated the other dogs and killed all of the Americans save for two people, one of whom managed to destroy it and prevent its escape. The name of this man was MacReady. It is possible the other survivor, a man named Childs, was a Thing.
Powers & Abilities
"This thing doesn't want to show itself; it wants to hide inside an imitation. It'll fight if it has to, but it's vulnerable out in the open. If it takes us over, then it has no more enemies. Nobody left to kill it. And then it's won."
The primary ability of the Thing is its ability to assimilate other organisms in order to survive. The true original form of the Thing is unknown as it has already assimilated other lifeforms not native to this world. Once it has assimilated an organism, it is not "stuck" to that one form. It can shape-shift itself into a patchwork of body parts and appendages based on previous lifeforms that is has assimilated. The Thing is also very strong and has a high level of stamina.
In order to assimilate an organism, all it needs to do is send one of its own cell into the victim and that one cell will replicate and assimilate the host organism. The Thing is also capable of detaching its own body parts that are able of thinking and moving by themselves. Likewise, different Thing organisms are capable of fusing with each other.
The smaller the Thing is, the less intelligent it will be. Small parts of the Thing, like a petri dish full of its blood, are intelligent enough to try to defend itself but not intelligent to not know when to and when not to defend itself, giving themselves up when they are attacked in anyway possible when it would have been better to do nothing and remain hidden as shown by the test developed by MacReady to "sniff" out any people who have been assimilated by the Thing.
Another example of diminished intelligence with diminished size is when the head of a assimilated man detached itself and went into hiding only to reveal itself by crawling out of its hiding space in front of the other men that were hunting it, showing that it did not know enough that it would have survived by remaining in its hiding spot.
Although the Thing is able to heal itself of massive injuries that would be normally fatal, it is not invulnerable. Flame and heat are capable of destroying it and intense cold will place the Thing in a state of suspended animation.
When a Thing has fully assimilated its victim, it will also gain that host's memories and knowledge, allowing it to more easily blend in. The Thing however cannot mimic inorganic material.