M12 Chaingun Warthog
From Halopedia, the Halo wiki
Warthog is a generic term for a lineage of United Nations Space Command ground vehicles. It is affectionately known as the 'Hog by UNSC soldiers.
Introduction
The Warthog is the modern-day "Jeep": all of its variants are equipped with a seemingly full-time 4x4 transmission, the ability to turn its front and rear wheels to allow for tighter turning, and can carry 3 people; 1 driver, 1 gunner, and 1 passenger (with the exception of the Warthog APC, which can carry 11 passengers, as befitting with its classification). The driver has no forms of attack except ramming enemies with the Warthog itself, which instantly kills an enemy on foot in Halo: Combat Evolved. In Halo 2, factors such as speed and direction do not allow every ram to be a guaranteed kill. The passenger, or shotgun seat, is the worst spot to be in, it is very difficult to aim (made even harder in Halo 2 with the removal of the first person view in the seat) while the warthog is moving, and if you try to reload, you will automatically jump out if you hold the X button too long. The gunner stands on the back of the Warthog and uses that particular variant's main weapon, may it be a chaingun, gauss cannon, or rocket launcher.
A Warthog variant is set to be a vehicle again for Halo 3.
Variants
M12 Warthog LRV
The M12 Warthog LRV (Light Reconnaissance Vehicle) is the most common type of Warthog, and is seen in Halo: Combat Evolved, Halo PC, Halo 2, and is slated to be in Halo 3. It mounts an M41 Light Anti-Aircraft Gun.
M12A1 Warthog LAAV
The M12A1 Warthog LAAV (Light Anti Armor Vehicle) is only seen on Halo PC, and mounts a rcoket launcher.
Its role as the primary UNSC LAAV is usurped by the M12G1 Warthog LAAV in Halo 2.
M12G1 Warthog LAAV
The M12G1 Warthog LAAV, its turret is a Gauss Cannon that acts like a miniature Magnetic Accelerator Cannon and fires fast-moving projectiles, takes about a half of a second to reload and has unlimited ammunition like all vehicle-mounted weapons systems. Before the launch of Halo 2, it was hinted by Bungie that certain types of warthogs would also make it in the game: a moon variant, cargo variant, jungle variant, and snow variant.
(The Warthog in Halo 2 is much more maneuverable than in Halo with less sliding and an E-brake (left trigger) for easier stopping and tighter turns, which the Halo 1 Warthog seems to be doing all the time. It also has a horn (right trigger) which can be used for various reasons. Namely, taunting.)
The Warthog seen in the Halo 3 Multiplayer screenshot seems to be the same, but most likely (like everything else) will be slightly different in it's appearance/features from those in the other games.
