The Laer were a xenos species originally from the world of Laeran. They were a serpent-like species that worshiped the Chaos God Slaanesh. They became extinct during the 31ºst millennium, at the hands of Fulgrim and the Emperor's Children Astartes Legion. Not to be confused with the Sslyth. And yes, GW made two snake-like aliens with multiple arms and expected nobody to notice.
The Laer had a physiology similar to snakes, with elongated and reptilian bodies, muscular thoraxes and ophidian-like heads, with insect-like pairs of eyes and very flexible jaws, sometimes used as weapons in times of war. They also displayed two pairs of arms. The Laer were very fond of genetic engineering, and their society relied heavily on particular individuals modified after birth to fit certain roles. As such, it was very frequent to find wildly different Laer: some with wings, some with gills and fins, some extra big as soldiers... Their displays of genetic fuckery were so impressive that Fabius Bile became inspired by them, and started making new Astartes organs based on what he learned from them.
During the Great Crusade, an expeditionary fleet of the Emperor's Children discovered the Laer species on the planet of Laer. And just one planet. Apparently, even though the xenos were highly technologically advanced and quite capable of space travel, they didn't have more than a single planet, their homeworld. It was a world of one giant ocean, with all of the Laer living on giant artificial floating islands suspended by hover-tech that the Mechanicum really wanted. Assuming they would be easy targets, imperial forces attacked the world, and were immediately curbstomped. In fact, the fight against the Laer was so difficult that many imperial authorities considered offering the Laer the possibility of becoming a protectorate, so that they could learn from their tech (though, in all honesty, it was mostly to gain some time before consolidating their forces and exterminating them). The Primarch Fulgrim, hearing this, became offended and swore to kill the xenos vermin himself, and so he launched his legion against the aliens. He was told it would take decades to do, and so shot back that he'd have them all dead within a month. After a short but brutal war with horrendous losses on both sides, the Laer were all killed within those 30 days, the last of the species fighting to the last snake defending their temples. However, Fulgrim observed the motion of their islands and determined which one was the "central" one that all the others moved around, making sure to save that one for a final ground assault led by himself. During this assault, Fulgrim charged into a strange yonic temple...
It is known that the Laer practiced the worship of the Chaos God Slaanesh, having massive temples dedicated to it, where they practiced all kinds of ritualistic orgies and many other dark acts of absolute depravity. The places were so corrupted that just watching them made people fall to Chaos. It is in the central of these temples where the Blade of the Laer was found, a ritual weapon possessed by a daemon. It was this sword that was the one item that started the corruption of Fulgrim during the days before the Horus Heresy.