Spiders are the most common and iconic of the Arachnid family of invertebrates, distinguished by their eight legs. Some species actively chase their prey down, others ambush them from hidden dens, and others still subdue prey with elaborate web traps, but 99.99999999% (except Uloboridae) use powerful venom to slay their prey.
People find Spiders very fucking creepy, and as such monstrous versions of Spider have long been a staple of /tg/ and approved /tg/ media. Usually by the simple extent of taking a Spider and making it way, way bigger than people are comfortable thinking about, or fielding them in an all-devourering Swarm with the singular goal of OMNOMNOMING, or both. The largest Spider actually alive today is the comfortingly-named Goliath Birdeater, which only reaches a one-foot legspan, and despite the name rarely eats Burbs.
In the Old World, Forest Goblins ride Horse-sized Spiders as cavalry, as well as strapping howdahs to the backs of giant Arachnarok Spiders, which can either mount catapults for mobile artillery, or act as fighting platforms for Wyrdgrots.