No one whom I have asked seems to know if the flying saucer at the beginning was meant to have been the Thing's, or had it either stolen it or hijacked it from another civilization? In the film although they explore the now partially excavated saucer at the Norwegian base, they never discuss what they find.
share1. In the 2011 prequel it's confirmed to be The Things.
2. In the 1982 film when The Thing is in control of Blair, it's trying to build a new spacecraft/'flying saucer'. That gives credence to The Thing being a spacecraft utilizing alien form.
There is another scenario : That ship was Alien explorers who go around taking lifeforms from other worlds. The Thing managed to break free. It battled with the last crewmember, who then crashed into Antarctica.
By absorbing the crew, the Thing gained knowledge on how to build a ship.