Bungie has posted several pics of Halo 3's Mongoose ATV and more info on a fake vehicle than anyone could ever need on their website.

"The M274 Ultra-light All-terrain Vehicle provides an alternate means of transporting messages, documents, and light cargo between units. It may also be used to transport forward observers, reconnaissance personnel and sniper teams. Its proven versatility has made it a vital asset to field commanders when used as an alternate means of communication."

Much more HERE