Reading about Halo "Hunters" today.... and came across this.
Azathoth117 asks: Are the orange worm-type thingies in the back of the Scarab the same things as the worms in Hunters, or is it just superficial resemblance?
Bungie: The resemblance is not just superficial. A future DLC map might expand upon that mystery.
So this means two things.
1) From what I've read, the "Hunters" are made up of thousands of little "Eel" like creatures that bind into a super organism. Usually this is the "Hunter" but in Halo 3, they also take the form of the Scarab (that big purple tank what kicks your ass), which appears unmanned, but is actually controlled by the eels.
2) There will be a Downloadable Map in the future which will take place on the "Eel" planet, or will have a Scarab to battle in/upon. I would point to the already mentioned "Purple Reign" Purple = the color of the scarab , Reign/Rain = the type of death which comes from above.