Deutoros-pattern Scarab

Summary
The Scarab is different from most of the Covenant's other ground-based vehicles. Instead of using anti-gravity pods, it walks about on massive legs. It is highly mobile for a vehicle of its size, and sports two turrets that shoot large red plasma similar to the Shade, as well as a large, forward-firing plasma cannon. This cannon is also somewhat reminiscent of the Gravity Cannon and the hunter's modified Fuel Rod Cannon.

The gun is extremely powerful, and is not a practical application for battle, except in battles that involve tight spaces such as cities. Rather, the Scarab is used to melt away miles of rock in order to excavate, no doubt for the structures and artifacts of the Forerunners.

While commonly thought of as a massive behemoth of an attack vehicle, it's official purpose, and name, is Covenant Scarab Mining Digger. Contrast to it's appearances in Outskirts/Metropolis and The Great Journey(Halo) of Halo 2, the Scarab's main purpose is to dig and excavate for the Covenant, probably in search of Forerunner artifacts. This would explain the nick-name it has earned; A scarab is a beetle that is able to dig extremely deep. Likewise, the Scarab is much better equipped to fire on a still target such as an obstruction rather than to fire on a large group of attackers all at once.

Appearances
During The Battle of Reach, Spartan-104 describes seeing several large "insect like" excavating machines digging up CASTLE Base. He destroys them using Plasma Mortar from a Wraith tank.

When the Covenant first travel to Earth, led by the Prophet of Regret, they believe the Human homeworld was simply an abandoned Forerunner world, and so they do not expect to meet Humanity on it's own homeworld. Since it is dropped orbitally, it may have hit solid ground in Old Mombassa, with the intent of searching for an artifact. Caught off guard and forced to fight on the human's terms, the Scarab is repurposed to crush any resistance in it's path. It is eventually cornered and destroyed by a group of Marines with the help of the Master Chief.

Another Scarab was on Installation 05. It was comandeered by Sergeant Johnson and used to break into the Control Room of the Halo.

Scarab Gun: Secret Weapon in Halo 2
The Scarab Gun is a weapon that players can pick up; it can be found on  several of the levels where the scarab is encountered.

Weaponry
(2) Mounted Anti-Aircraft Turrets

Ammo: Super-Heated Plasma, (possibly have needler homing technology) Rounds Per Minute: 400-600 rounds/minute Notes: Presumed Computer Controlled, but a close look indicates that it may be manually controlled additionally.

(1) Front-Mounted Cannon

Ammo: Super-Heated Plasma, presumably un-guided. Rounds Per Minute: 10 second stream; 30 to 40 per second Max-Temperature: ? Movement of cannon: Through careful examination, one can see that the inside gun part seems to move forward from a cradle as the flaps open up. Notes: The projectile emmited from the Scarab's main gun emits a neon-green/bluish glow, indicating extremely high temperature. It's force is enough to destroy any combat vehicle in a fraction of a second, and could presumably melt through an interstellar war-ship's armour with the ease of a Covenant's capital ship's plasma bomb.

Physiology
Height (standing, toe to highest point.): ? Width (standing, front to rear toe): ? Length: 64.9 Meters Weight (unloaded): ? Speed (unloaded): ? Materials: ? Shielding: No energy sheilding is known of. Armour Alloys non-identified.

Other
Deployment: Orbital Drop. Crew: At least 5 Elites, a white-armored commander among them. Grunts and Jackal are seen in greater numbers above and below deck.