The MLX is a civilian vehicle, seen in the Halo 2 levels Outskirts and Metropolis. It looks like a normal car, but with the 2 rear tires being supsended on "legs" independant of the car, meaning the engine is likely housed in the rear. It is not pilotable, and can be destroyed with sufficiant weapons fire, making it a possible hazard for units within the blast radius.
http://nikon.bungie.org/misc/sloftus_scalecomparison/?display=UNSCVehicles