Rhagodessas, aka "Proof Positive that Demon Princes of Vermin are utter bastards", are an ancient monster from Dungeons & Dragons, having first appeared in the D&D Expert module "X1: The Isle of Dread".
To picture a rhagodessa, take a solifugid - better known as a Camel Spider - and then scale it up until it is at least the size of a small horse. Then try to scrub the image of that coming for you at a speed of way-too-fucking-fast from your brain. These giant arachnids are truly some of the most visually horrifying of their kind outside of the various demonic spawn of Lolth.
What? Alright, alright, we'll get serious. The rhagodessa is a primeval arachnid the size of a small horse. Its hairy, dark brown body and its oversized yellow head are dominated by a massive set of pincerlike mandibles, which account for nearly a third of its body weight. It has four pairs of segmented legs, as well as a front pair of shorter appendages that end in pedipalps studded with dozens of hooked suckers. The rhagodessa catches its prey with these pedipalps and then crushes the creature in its jaws. Anyone so grabbed will find it nearly impossible to escape. Once a rhagodessa has secured its prey, the victim is dragged back to the creature’s nest and liquefied for later consumption. Rhagodessas can climb just about any surface, even while carrying prey.
According to its 4e lore, these arachnids favor arid or tropical environments, and are particularly likely to inhabit lands shared by other "primeval" beasts. The Isle of Dread is reputed to have specimens of rhagodessa which are much larger than their already terrifyingly oversized "common" relatives. They are relatively territorial; whilst they have been known to cohabit in small broods of up to half a dozen, once a rhagodessa finds itself some suitable hunting ground, it likes to stay put and will fight to keep what it considers its own. Combined with a surprising receptiveness to conditioning, and rhagodessas actually can be trained to grotesquely effective guardians.
4e lore also states that rhagodessas are extremely popular with blood sports arenas because of that combination of trainability, hideousness, and innate bellicosity. The species has a peculiar enmity for other species of giant arachnid, especially giant scorpions, and such arenas can make a mint off of matches between the two. Of course, most sane cities don't want these aggressive and dangerous creatures anywhere near their borders, so trade in rhagodessas is illegal and mostly done through black markets and smuggler's ring. Farmers in the wilderness regions bordering rhagodessa territory dread the annual mating season, as the larger females prepare for their month-long fast as they rear their offspring by devouring up to twenty times their own weight before the mating season proper. As a result, they become much more active, roam further, and are much, much more aggressive; the harsh, rattling sound of rhagodessa mating calls filling the air is actually a relief, as it means an end to their depredations.
Rhagodessas burned such an image into the brain of players of the original Isle of Dread module that they were brought back for the Isle's return in the Savage Tide Adventure Path in Dungeon Magazine. They would then receive a 4th edition update in Dragon Magazine #418, whilst Pathfinder would bring them in under the copyright-exempt name of "Giant Solifugid" in their Bestiary 2.