Continuity mistake: When Palmer is revealed to be a thing, he shoots up to the ceiling. Windows is ready to use the flamethrower on him. He has the flamethrower pointed up to the ceiling, yet when Palmer falls to the floor, in that split second, Windows changes into a completely different position with the flamethrower. (01:23:35)
Continuity mistake: When Childs takes an ax to the supply room door where MacReady has broken into, there is a shot of him hitting the door high up. Then there is the shot of MacReady almost frozen. When the shot cuts back to Childs, the ax marks in the door are not consistent with the shot from a few seconds before. (01:12:50)
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.