Harpies are not very detailed in the game, but has a close resemblance to the Onoskelis; their torso and head are the same as the Onoskelis, but the arms and legs are of an hairless bird/bat. They act as scavengers as they can be found eating only dead corpses; both demons and damned souls alike.
Harpy acts as an vulture, and only scavenges dead things rather than attacking living things. They pose no threat, as shown when seen by them or getting to close to them; makes them fly away in fear.
In mythology, harpy is an roman/greek creature that is half female half bird. But it can be notable by the Red Goddess that she taken most of what Nimrod has created, and the deepest part of Hell an resemblance of his own kingdom. It can be predictable that she might as well took some mythology creatures from other lands; to mimic they're creatures into her own creation.