Other mistake: The brief chess game (four moves) shown in the beginning of the movie makes absolutely no sense. For instance: There are no white bishops on the board, but after the human player taps a few buttons, the computer says that he just moved one. Within the two moves described first, the positions of the pieces change completely. In addition, some pieces are now gone and there are new pieces on the board. It's like an entirely different game. The final move is said to have been made with a black rook, which is impossible. In the shot shown before this move, there is not a single position available to that chess piece that would result in an instant checkmate. Nothing in the editing indicates that these few short shots are supposed to be viewed as separate games. (00:04:50)
Other mistake: When Blair goes crazy with an axe, they put him in the tool shed and Doc injects him with something to calm him down. The problem is: Doc injects him in the inner elbow, but wipes the upper arm as if that's where the injection went. Plus you only inject in the inner elbow when you want something IV, but there was no tourniquet, no searching for a vein. It should have been given IM, into a muscle, like the upper arm.
Other mistake: When Doc's arms are being ripped off by Norris' chest, his left arm is cut by the teeth but his right arm is pulled off above the wound like rubber. (01:13:00)
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.