Harpies are fast flying monsters that often appear in deserts. Because your movement speed is lower there, they will get 2x turns often, not suffering the penalty due to flight. That said, they are still fairly weak and low-health targets.
Harpies are most dangerous when you have the Loyal Pet perk active. A harpy that lands two successful hits may be enough to kill your pet before you even have a chance to react.
Has an extremely low chance of dropping Save Crystals.