What is the watcher in the water? How did it get there?

The watcher gets its first kill that we know of when Balin tries to retake the mines. (If anyone knows of earlier references, please let me know.) During that attempt, the watcher gets Óin. 25 years later, it's woken up by Boromir and it nearly gets the fellowship, but instead traps them inside the mines.

Gandalf seemed to think it was one of the nameless things that lives beneath middle earth which gnawed its way to the surface. What other nameless things are there that have reached the surface?

by Joan Wyatt