Special optical effects supervisior Peter Kuran discusses the title effect, from the documentary The Thing: Terror Takes Shape:

When I did the effect for the title I used…a fish tank that was about four feet wide by two feet high. I put smoke in the fish tank and on the back of the tank I put the title that was drawn on an animation cel and behind that I had a piece of plastic garbage bag which I stretched over a frame and behind that I had a light pointing through the letters. When I photographed it, I put a flame from a match to the plastic. The plastic would open up and let the light through the letters. That is how the letters look like they form and burn on with the [light] rays. It was a simple process but we went through a lot of takes; one take only formed the letters “N.G.*”

*On a film set, the mark of “N.G.” or “N.F.G.” is taped to any equipment that does not work. No good.