Keelbug

The Keelbug is a deleted Covenant race seen in the Halo 2 Collector's Edition Video. It was supposed to come into the battlefield, cut dead bodies into sections, and then fly off with them, as a way of cleaning up the bodies.

In Halo 3, the Drones can at one point be seen cutting a Marine in two, and flying off with him, so it is possible that in Halo 3, the Drones fill the role that the Keelbug was intended to fill in Halo 2.