"It has a quickness about it that seems unnatural for its large size, yet its hunger is about right."
- – Endril, Kalonian naturalist about the Giant Spider
"Get back you eight-legged freaks!"
- – Random member of the Adeptus Astartes during their first contact with the Megarachnids
The Megarachnids are a xenos species discovered on the planet Murder (real subtle there GW) during the 31st Millenium. They are sapient, arthropod-like creatures who were stuck on their home planet after a war with the Interex, and were accidentally found by the Imperium at the final stages of the Great Crusade.
It is believed that, in the distant past, the Megarachnids had some sort of limited galactic empire, but after a war with the human Interex, they were beaten quite badly, and were forced to give up their means of space transport, as well as being contained on their home planet of Murder. The Interex, being basically 40k-Star Trek before Horus had his little tantrum, could have eradicated the species there and then, but they designed a series of measures to make sure Murder became a safe place for their defeated foe to live in, as well as to avoid them getting out. That way, if anything goes wrong and no one is left who can stop them, the freaks could wipe out all life. But hey, at least the Interex can feel good about themselves for not
saving everyone murdering the poor harmless spider monsters. One of the measures was a series of warning signals surrounding the planet, which were promptly ignored by the Imperium of Man once they arrived there. After a six-months war, the Megarachnids were in some serious trouble, being borderline extinct and all, but the Interex managed to calm down the Astartes just enough to start diplomatic means to solve the problem. After this, the Horus Heresy happened, so we can safely assume they got exterminatus-ed during the crossfire.
Megarachnids have a disposition similar to... spiders. They have eight legs, four dedicated to locomotion, and four for attack and defense. Their claws were extremely hard, being covered in a strong metallic substance that could pierce the ceramite armour of the Space Marines. Their bodies were covered in this substance as well, which meant they were tough cookies to crumble, able to ignore several bolter rounds (and there's no info on whether Kraken had been invented yet). Megarachnids were blind, having no eyes at all, but they had their other senses heightened enough to be able to hunt with precision. Also, instead of shooting web, they shot cement, resembling a scaled-up adhesive-firing Terran arthropod.
Maybe there's a possibility of being a tyranid offshoot? Not every bug in the galaxy has to be a Tyranid!
Their behaviour was similar to eusocial insects, with different kinds of specialized breeds, although no queen was detected. There were, however, Megarachnids the size of titans, which also had a lot of importance on their hierarchy. Also, they had so many eggs at a time that Astartes just gave up trying to destroy them all.
Despite them being a species with very biological-based technology, they had some very interesting tools, such as trees (made of the stone they secreted) that could disrupt electromagnetic signals, or ways to control the weather at will, both used heavily during the Space Marine invasion.