Slaangors, as their name suggests, are Beastmen in Warhammer Fantasy, Warhammer 40000, or Age of Sigmar who have dedicated themselves solely to the worship of Slaanesh and been rewarded with his/her Mark in the process. Like the other "Marked Gors" units, they are an ancient concept, dating all the way back to when Beastmen were introduced in "The Lost and the Damned", the second book of the Realms of Chaos duology.
In Realms of Chaos, it is said that a Slaangor typically has white (or near-white) fur and pale or pastel skins, as well as frequently having green and/or saucer-like eyes reminiscent of a Daemonette. They are also most likely to resemble Bovigors, with the heads and horns of cattle, ala minotaurs, but theoretically they could have the form any animal associated with sex and sensuality; deer (stags), horses, cats, rabbits, foxes, and snakes would all make logical alternatives.
Old Slaangor models give them one breast and one flat chest like current Daemonettes, albeit with the breast uncovered, and notably one has the head of a unicorn while another had the head of a fish. Many of them are wearing garters or thigh-highs.
For most of WFB history they didn't really have their own stats, and existed as models (until they were squatted and replaced with generics for all varieties) that represented what Marks your forces had.
In the original Realms of Chaos duology, when introduced in "The Lost and the Damned", the only mechanical bonus for being a Slaangor was Hatred (Worshippers of Khorne) and gaining Slaanesh spells of your level as bonus freebies if you were a Slaangor Shaman.
In the Beasts of Chaos armybook, the only Slaangors were Bestigors, chariots (ridden by Bestigors) and Characters, since only they could take the Mark of Slaanesh, presumably because they were the only ones to prove themselves. In all cases the Mark made whoever had it Immune to Psychology and given Beastmen Leadership (and the items it unlocked), that wasn't bad.
Variety was gutted when Chaos was split into three armies and somebody took Marks away from the Beastmen, then retconned the fluff to say that Beastmen were never given and could never have Marks.
Beastmen mutations are not automatically Chaos in 40k, but at the same time Beastmen aren't a dominant and uncontrollable force so most Slaangor are just considered Chaos Mutants and lumped in with human Cultists. Way back in the day, when Chapters were first being created and released, each one would get a list of how many Beastmen slaves they had access to; obviously any Chapter of Chaos Space Marines that was devoted to Slaanesh had Slaangors.
AoS Slaangor are tsundere for civilization, loving the shit out of the things that Beastmen ordinarily hate like soft luxuries and things of gentle beauty, and are overcome by massive waves of guilt and shame as soon as the novelty has worn off which they bury under even more frantic hedonism to avoid feeling anything but satisfaction despite the building wall of self-loathing. So basically they're the most realistic Slaanshi lore we've ever gotten.
They feast, fight, ferment and fuck until they're out of food, wimps to pick on, booze to drink, and holes/phalluses and have to find a new place to party, going full Noise Marine on the Brayhorns as they tear off with reckless abandon to destroy the defenders of whatever poor settlement they're swarming as quickly as possible so they can get to partying hard. These frenzies are called "Slakefrays", where they drink, rape (implied), eat, and kill fucking everything they can. Unlike most Beastmen they stick around when there's nothing left to kill, trying to squeeze every last bit of pleasure they can from whatever is around. The most prized things like gold and jewels are as likely to be melted down to decorate their horns as they are to be fucking eaten for the same reason rich assholes in real life eat gold leaf and diamond dust meat and candy (don't ask us, our money goes towards Citadel brand plastic crack). They wear the most exquisite fabrics, or decorate their bodies and horns with them. Baubles are hung and carried, magic runes and items kept for no other reason than to show off, and the Herdstones of the Slaangor are so ornate they may as well have been made by a civilized race of creatures with access to way too much gold (so
Any gathering of sufficient size of AoS Slaangor is called a Depraved Drove, which is the name of their army type in the crunch.
The Direchasm starter box for Warhammer Underworlds appears to introduce a proper Slaangor called Slakeslash, who in addition to the classic pale skin is differentiated from a normal Gor by his Daemonette-esque crab claw. And looking like he's dressed in leather bondage gear, of course.
Slakeslash was a precursor for the new Slaangor Fiendbloods which are being released alongside some new Slaanesh mortals. They are distinctly bovine with gilded horns and BDSM gear.
A particular band of Beastmen that has swelled in numbers beyond reason, that topples the largest of civilizations with ease. Carving a path of destruction through Aqshy, they've gone on to such heights of depravity that they've been joined by Daemonette bands of Seekers of Slaanesh who think that the Beastmen know where Slaanesh has been hidden and are being drawn to him; in reality, the Gravenkin are just that insatiable that the Daemonettes can't comprehend it.