The giant spider, s, is a monster in NetHack. They are faster than you and have a rather damaging poisonous bite; however they are too large to be able to hide as some of their kin are capable of. This does not stop NetHack from giving them an item to hide under if they are created with the level though, just like the other snakes and spiders who can hide. Their corpses are poisonous.
If you are polymorphed into a giant spider you can spin webs using the #monster command without using up any power. Spinning a web inside of a monster with a digestion attack will cause it to spit you out (webs do not bother other engulfers). Spinning a web can be used to remove a (spiked) pit, hole, trap door, or rolling boulder trap. This is done without creating a web on the same square that must be removed. Since most other traps can be removed by digging a pit over them, it is possible to permanently remove almost any trap.
Every web trap is generated with a giant spider, and most (3 out of 4, or all of them if you have the Amulet of Yendor) giant spiders generated with the level are given webs. Therefore, you will usually, but not always, see webs and giant spiders together.
In dNethack, giant spiders can grow up into mirkwood spiders, spiders with a web attack and poisonous bite.
Eight legged creature capable of spinning webs to trap prey.
"You mean you eat flies?" gasped Wilbur.
"Certainly. Flies, bugs, grasshoppers, choice beetles,
moths, butterflies, tasty cockroaches, gnats, midges, daddy
longlegs, centipedes, mosquitoes, crickets - anything that is
careless enough to get caught in my web. I have to live,
"Why, yes, of course," said Wilbur.