Harpies spawn in mountainous biomes on uncovered dirt. They will also spawn on top of exposed dirt in skybases, such as a roof.
Harpies can break glass and glass panes, making them a mortal enemy of skybase glass roofs and windows. Reinforced glass such as Quartz glass is advised - while not immune to harpies, it is stronger than normal glass.
When killed, Harpies typically drop feathers, as well as having a small chance to drop a gold ingot.