From Destinypedia, the Destiny wiki
The Harpy is an airborne Vex unit capable of maximum speed and versatility.
Harpies fly rapidly in and out of combat. They must stop and stabilize before attacking, opening up their shells and revealing a writhing mass of filaments. They are equipped with Harpy Slap Rifles which, as its name suggests, is a weapon exclusively for the Harpy. In Destiny 2, Harpies employ the usage of a beam weapon fired from their bodies instead of a Slap Rifle. The meaning behind this modification remains unknown.
Harpies are highly aggressive when fought, often dodging and maneuvering around gunfire. They tend to congregate around Guardians in order to overwhelm them, and are highly dangerous in numbers. Harpies in Destiny use Slap Rifles, while those in Destiny 2 employ the Vex Laser Rifle.
List of appearances