John Carpenter's "The Thing"

[Hoots Rowlet]

Okay... Let's say someone wanted to work this creature into their games.

How would you go about working the Thing?

[Brainstem]

Maybe give it Mimic, Duplicate Self, and Shapeshift with a Challenge that it has to kill the target it mimics before it can take on the new form.

You could also rule it as a possession. I don't have my books on-hand, so I don't remember how the power works, but include something to require in there that you successfully injure the opponent (if not successfully kill it outright) before being able to possess the body and also add something about being able to maintain multiple possessions.

Things you'll have to decide that aren't displayed on screen:
-Do all the mimics share the same consciousness? If so, that can help point you in the right direction for powers.
-Does the Thing kill the target before possession or does the act of possession kill it? In the scenes where they're observing the blood, it looks like it's a slow going take-over.

Honestly, though? I would just GM fiat a lot of it. The possessions all occur off-screen and you should maintain that. Stat out the monster and just use those stats, minus any possession abilities. Just fluff it and say that, if caught alone with the Thing and it wins in a battle, the player is taken over, but really that shouldn't happen because that's such a lame thing to do to a player.