The origins of such creatures is unknown. The Neekerbreekers are first heard as Strider leads the hobbits to Weathertop; their journey takes them through the Midgewater Marshes. There in the marshes, the group decides to make camp. As night falls, most of the group find themselves unable to sleep because of the "Neekerbreekers," as Sam called them. It's quite possible that these creatures are really Katydids or some other locust from the Acrididae family.
They are described as the evil relatives of the cricket, thousands of them that go "neek-breek breek-neek".