These helicopter gunships are a derivative version of the AH-64 Apaches that were used by the Brotherhood in the First Tiberium War and were been developed as their predecessors had grown inferior in light of the Global Defense Initiative's newly developed Orca Fighters as well as increasing local anti-air capability.
After the start of the Second Tiberium War, Black Hand leader Anton Slavik took his loyalist forces to combat those of the rebel leader General Hassan. During one of his missions, Slavik rescued a captured Nod commander that was trapped in one of Hassan's prison bases and used a Harpy to evacuate him from the facility. Before, at the onset of the Firestorm Crisis, Slavik asserted his dominion over the Inner Circle by having his agents assassinate rival generals; one of whom was traveling on his Harpy when it exploded.
These gunships were later retired from service from Nod's arsenal as they had become obsolete and instead work began on Vertical Take Off and Landing, the technology used to create the Orca. This led to the creation and the deployment of the Venom attack ships which were used in the Third Tiberium War.
The Harpy features a much more aerodynamic, stealthy design, a powerful rotor and an integrated chaingun cannon in its forward section. It is mainly a scout, used to patrol areas or evacuate important personnel (the gunner isn't an integral part of the crew). It is also the bane of infantry, as its chaingun cannon can easily slaughter advancing infantrymen.
Unlike the Apache before it, it does not carry rockets nor missiles. However, it has two 30mm chain guns as opposed to one on the Apache, giving it twice the firepower. Despite this being the case, they were no match for the Hover MLRS or the Mammoth Mk. II walkers.
The basic air unit of the Brotherhood in Tiberian Sun, the Harpy is a pretty weak unit, though useful for taking out infantry or harassing other units. Since it is also invisible on the mini-map, it's useful as a decoy or distraction.
Despite the fact the Harpy has rotors in renders, there are none present in-game, due to engine limitations.
Behind the Scenes
The Harpy is the name given to a horrible half woman half bird flying monster from Greek mythology which harassed its enemies from the sky.