Keelbug
From Halopedia, the Halo wiki
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 himTemplate:Fact, so it is possible that in Halo 3, the Drones fill the role that the Keelbug was intended to fill in Halo 2.