From Halopedia, the Halo wiki
The Keelbug is a deleted Covenant species seen in the Halo 2 Collector's Edition Video. It was supposed to come into the battlefield, cut up dead bodies into sections, and then fly off with them, as a way of cleaning up the bodies. They were originally going to be used in Halo: Combat Evolved as a clever method of removing NPC corpses to free up memory. This race would have been a gruesome addition to the Covenant.
The concept of an insect-like creature as an enemy may have led to the design of the Yanme'e that later first appears in Halo 2.