M274 Mongoose

Summary
This vehicle did not make it into the final version of Halo 2, but is showed in the "behind-the-scenes" video on the Limited Edition version of Halo 2.

The Marines' fastest vehicle is the Mongoose All-Terrain Vehicle. Essentially four wheels and an engine, this ATV can travel very fast over nearly all types of terrain. With no weapons on board, the driver must use his own weapons for defense, though this means taking one hand away from the vehicle's already precarious controls. Due to this, the driver would use a Magnum or an SMG if he could use it with one hand. It serves a similar purpose of the Covenant Ghost.

Even though you cannot use the warthog's baby-brother mongoose you can find it in a few maps made by halo fanatics in the widely popular Halo: Custom Edition for the PC.