The Wild Hunt is a mystical organization that appears in various forms throughout myriad European mythologies. At its core, the Wild Hunt is a supernatural hunting party that can be seen racing through the wilderness at light, although details such as who partakes (fey, fiends, ghosts, or some combination thereof have all been cited), who leads the hunt, and what it actually hunts depends on the precise tale that you hear!
As a key bit of European mythology, the Wild Hunt has made appearances in both Dungeons & Dragons and in Pathfinder. Its ties to faerie-lore has seen it appear in Changeling: The Lost as well. It is also a major part of wood elves lore, heavily related to Orion.
The Great Wheel version is covered in Dragon Magazine #342, and is... not very well detailed; the rear-section of the issue dedicates its Divine character option to presenting the Master of the Wild Hunt as a deity that can be worshipped by clerics. He is described as a Neutral (leaning towards Neutral Evil) deity mostly worshipped by druids and rangers, who exists to hunt the most worthy prey imaginable; Wild Hunts in the Prime Material are typical sects of his mortal worshippers, and usually have more specific quarry.
The World Axis version is first mentioned in the pages of the Monster Manual 2. Here, it is explained that the Firbolgs, a race of fey giants, undertake Wild Hunts as a sacred rite, seeking to prove their worth to the three goddesses whom they worship and to punish oathbreakers. Dragon Magazine #428 would subsequently explain that the Wild Hunt of the Firbolgs is a pale imitation of the true Wild Hunt, which is led by an outcast archfey named Cerunnos, the Horned Lord. The "true" Wild Hunt comprises of many different Gloaming Fey, as well as firbolgs and centaurs, aided by undead (especially vampires; an eladrin vampire demigod named Kannoth was once a member of the Wild Hunt himself) and warlocks.
In Ravenloft, the Wild Hunt, while superficially similiar to the versions above, is a manifestation created from the mists by the Dark Powers. They most often manifest in Forlorn, but can manifest anywhere temperate, and from there begin searching for any intelligent being of any alignment, except True Neutral, for prey. Any being who sees the Wild Hunt after the prey has been chosen, is compelled to join the hunt, and often evil beings, such as hags, werewolves, fey, and undead, and PCs who've failed a Powers check are invited to join the hunt. Oh, and the Darklords are immune to the call and can't be chosen as prey.
Appearing in the 6th Pathfinder bestiary, the Wild Hunt of Golarion is a civilization of distinctive fey, who collectively owe loyalty to a god-like archfey called the Horned King, and whose culture revolves around hunting the greatest trophies they can imagine. Stats are presented for three specific varieties of "Wild Hunter"; the Scout, Archer and Monarch. Stats are also presented for their fey hounds and steeds, too.
Changeling: The Lost
The Wild Hunt of Arcadia is not one singular organization, but a phenomena; it refers to one of the True Fey going on the hunt, with an entourage of hobgoblins at their beck and call. A cursed fey artifact allows a changeling to summon a Wild Hunt to their presence, and is regarded as one of the very last things you should ever, ever have to use.
Basically the WAAARGH!! for the Wood Elves. Every midsummer, Orion would led a bunch of his forest kin to ass raped any of his enemies. Everybody that is part of the Wood Elves faction in Athel Loren would often felt their primal instinct arise at this period that they would either join it or avoid it. The notable enemy victim that gets assraped during this period are either the Bretonnian (because they live closely) or the Greenskins (lives in the nearby mountains but also got assraped in the first ever wild hunt when they tried to WAAARGH the Oak of Ages).