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.

Pathfinder

To troll with any monstergirl fans who might be into the setting, Pathfinder explicitly made its harpies rather good-looking (something that D&D didn't do until 4e), but gave them three very important negative traits.

1: They have absolutely no sense of hygiene, meaning they never wash or clean themselves. As a result, they stink to high heaven of blood, shit, rotten meat and worse. Fortunately, they can be taught to bathe themselves, and it's mentioned some harpies make dangerous urban predators simply by learning to get rid of their distinctive odor.

2: Being Chaotic Evil means they are very rough lovers, often outright sadistic; sex with a harpy is often a very painful, humiliating experience. Still not bad enough? There's trait number 3...

3: Sexual cannibalism. In Golarion, harpies eat their partners after they're done screwing them. This is usually done for shits 'n' giggles after just casual nookie, meaning you might survive a casual fling with one, but their messed up culture declares that any harpy who does not eat the father of their child is a weak punk of a slut. Which means that if the harpy wants a baby, the daddy is going to be a post-coital snack once she's knocked up.

All the same, they're still sapient creatures, and one adventure tree explicitly allows the PCs to take one on as a sidekick and help her get over her "bad habits."

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 during the mating season. Though they will let a mate go once their mating season is over unless it's a black harpy. 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. Harpies also have a rare chance of dropping a Large feather which can be used to make Harpy Wings in Hard mode which surpass the Angel and Demon Wings in terms of flight range. The Normal Feathers can be used to create Featherfall and Gravitation Potions, which when before you can get the Lucky Horseshoe, makes it much easier to navigate the skies to find more islands.

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.