So what was it?

It was an ancient life form that had been living under the earth for so long.....occasionally coming out to feed. That was how the disappearances of large groups of people had been explained (i.e., the Roanoke colony that disappeared) It also absorbed the memories and such of those it consumed, which meant it knew things they knew, and could take the form of anything it had absorbed. If you recall, there's a part in the movie, and the book, where they talk about flatworms learning how to go through a maze, then being ground up and fed to other flatworms, which would absorb the knowledge and be able to go thru the maze pretty quickly.

"Uh, and that's a problem because??????"