John Carpenter's The Thing.

If you've seen this movie, you know i don't have to give you the hard sell.  If you haven't seen it, are you crazy? Get it now!

Synopsis:

An American scientific expedition to the frozen wastes of the Antarctic is interrupted by a group of seemingly mad Norwegians pursuing and shooting a dog. The helicopter pursuing the dog crashes leaving no explanation for the chase. During the night, the dog mutates and attacks other dogs in the cage and members of the team that investigate. The team soon realises that an alien life-form with the ability to take over other bodies is on the loose and they don't know who may already have been taken over.