Even supposing the Thing didn't create the flying saucer it landed in, it has the ability to create a new one, so there could be Things all over the universe.
The Thing's strategy is more crude in the very beginning, simply attacking humans like a wild animal. When it finally assimilates a human, it starts scheming, getting people alone before attacking, hiding its trail and trying to reason with people. Not only can it now try to replicate people, but it knows how humans behave and has learned that they can kill it.
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