Character mistake: The last time Doc tries the defibrillator on Norris, he uses it on the abdomen instead of on the chest.
Character mistake: When MacReady is giving the blood test, the scalpel they are using is not properly sterilized after each person cuts themselves (shown when Nauls cuts himself, then when Windows goes next: all he does is wipe the scalpel on his pants). Using the same scalpel seems like an extremely stupid thing to do as Fuchs told MacReady that even a small particle of the Thing could take over a person. (01:19:55)
Character mistake: The spaceship is much deeper in the ice than frozen thing, indicating it was buried a hundred thousand years after the spaceship landed. Norris' calculation of the age of the spaceship wouldn't apply to the thing, which was hardly buried at all, by comparison.
Answer: At the end, the large creature presented itself as an amalgam of beings it had absorbed-part Blair, part dog, and various other beings with tentacles, insect-like legs, and a worm-like body. I don't believe that we really ever see what its true form is, if it has one.
Erik M.