Fang is a unit in Earth 2150.
The name "Fang" is actually a nickname for the soldier driving the alien vehicle; his real name is never confirmed. In-game, the name "Fang" refers to both the pilot and the vehicle.
The pilot is a veteran from the Great War. He calls the war "pure hell" in a recorded statement to the LC Council in which he asks for Asylum. He admits that the war made him paranoid; since 2140, he has never been without a gun.
His vehicle was salvaged from a military base that was once known as "Area 51". When the war flared up in 2148, he was selected to pilot the craft (the UCS were't able to build in a coputer system like with the rest of their army), despite his claims that "I'm not the best flying ace around - Rickenbacher had a dozen more kills than me, but these days the old guy needs at least a full tumbler of bourbon to stop his hands from shaking".
Fang was sent to the LC with a peace proposal from the UCS president. In return for an Alliance against the ED forces of Khan Vladimir II, the LC got an experienced soldier (something the LC severely lacked at the time) and Fang's buggy, which they were allowed to examine. In fact, the amount of experienced soldiers was so slim that Fang was immediately appointed head of the LC's military forces.
Later, when the UCS broke the Alliance, Fang remained with the LC and fought against his own "countrymen". He presumably boarded the LC evacuation ship, but has not been heard from since.
In-game, Fang is an extremely powerful unit, especially in the earlier missions, where he can destroy the entire enemy base by himself. His Plasma Cannon is also capable of disabling an enemy unit. Early on, the only thing that can harm Fang is aircraft, as he cannot target aircraft.
Later on, Fang becomes less and less useful, as his death means instant failure of the campaign. During the later parts of the campaign, ED lasers and UCS Plasma Cannons can make short work of Fang (even though he can be equipped with a shield in one of the LC missions), while deflecting his own attacks with their shields. Even other weapons like ED heavy Cannons become a real problem in later missions.
His only constant weakness is that he will quickly run out of ammo for his weapon, and, like other energy weapons in the game, these reload slowly, giving the enemy the time to recuperate and "gang-up" on Fang. One way to deal with this is once he is out of ammo, get him to retreat and finish off the enemies with other units