Harpy

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They have power over the loins of men.

A classic monster in Dungeons and Dragons, the Harpy is at once a perfect horror; beautiful and horrible all at once. Known for their ability to sing and draw prey, inexorably, towards them, they are notoriously sadistic and have a well-deserved reputation for playing with their prey in an excessively cruel way, only killing it when they've gotten their rocks off by subjecting their "plaything" to unspeakable torments.

Avian Dark Eldar for the win, really. They can pose a serious threat to low-level adventuring parties, except in 4th Edition, where they're kind of useless as anything but a speed-bump, like most monsters in 4E.

Living With Monstergirls and the Monstergirl Encyclopedia

While monstergirl harpies tend to be kinder and much more gentle than the classical sort, Harpies have a much deeper and more sinister reason for luring individuals off - they need males (preferably humans) to breed. A male caught by a harpy, suffice to say, has little hope of escape due to her singing ability. Competition between Harpies for mates is fierce, and a Harpy will go to extravagant lengths in order to impress a mate and win them away from a rival. In the Living With Monstergirls series, a Harpy's lover takes her out to the fields to exercise her wings every now and then, an activity which usually gets her amorous enough to enjoy "quality time" with her betrothed.

Harpies in Terraria

RUN, BILLY, RUN!! SAVE YOURSELF!!

Harpies are one of the more dangerous enemies in Terraria, though not one of the more powerful. Encountered at high altitudes far from the initial spawn, these free-flying fiends are largely considered Terraria's equivalent of Dwarf Fortress's infamous Carp in that they exist to ruin your shit.

With 100HP, they're one of the most durable normal enemies, and they tend to appear in large groups (often 5 or more; god help you if a Blood Moon's going on when you're in their territory). They do a huge amount of damage (nearly as much as Terraria's feared Demons), fly very quickly (making them rather hard to hit without long-range weapons), and fire volleys of feathers off in strafing runs as they zip around the area. They only appear high in the air (and even then, far from the starting spawnpoint), but they can turn any attempt to build at high altitudes into a frustrating and frequently-fatal endeavor.

Trying to build a curtain wall at the edge of an island whilst under siege from 4-5 of these fiends will result in an immediate need to roll for Anal Circumference. Whilst most well-known for their ability to knock you off ledges and bridges and sending you hurtling to a horribly mangled death on the ground far below from fall damage, they are far more dangerous if, god help you, one manages to get inside your base, where they can set about crashing into you, killing important NPC Shopkeepers, and in most ways doing a bang-up impression of a Dwarf Fortress Carp. That flies. And shoots.

Fucking Harpies.

Almost d'aaaw enough to make you forget that they want to slit you open and feast on your innards.

It's not all bad news, though - Players who build their homes high in the sky will frequently come to enjoy the safety and security that comes from having flocks of the goddamned things swarming around their base at all times, providing a constant diversion and harassment against any foe bold and foolish enough to approach in PVP maps. Harpies can't burrow or teleport (like a lot of monsters that otherwise occupy dangerous terrain in Terraria), so a walled-in area with reinforced walls will generally keep the fucking things out of your home, and provide you with useful guardians all in one stroke. Sadly, this means you need to actually build a base in Harpy territory, which is easier said than done.

Note, however, that once you reach Hard mode, the Harpy is no longer the premier threat of the game, especially if you build bases in the air. The Wraith is a far more ubiquitous and annoying threat, freely moving through walls, dealing ten fucktons of damage with each hit and easily obliterating your NPCs. Evil Clowns answer the age-old question of what could happen if there was a monster equivalent of a TNT-spamming jackass on your server, throwing bombs and destroying structures. But by far the worst of a very bad lot on Hard mode is the Wyvern. With over 4000 HP, the ability to go through walls, and one of the highest damage values in the game, Wyverns are the Carp of Terraria's Hard mode - such is their power, in fact, that they cause the Harpies to say "fuck this" and start flocking at lower altitudes; you'll start seeing them the second you crest a low hill.