From Rebirth of the Night Wiki

Harpies are flying hostile mobs that spawn in Overworld and Aether mountains and high places. While their basic attack is weak, they pick players up and drop them from the sky, dealing massive amounts of fall damage if allowed. Hitting a harpy once will cause it to drop you.

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.

A harpy on the attack.

When killed, Harpies typically drop feathers, as well as having a small chance to drop a gold ingot.