I think its more that people in general just dont like spiders i dont know why but to most people they give you a queezy feelig
rowlings spider was actually good as i recall except his/her children were little
ba---rds anyway the only myth i know of as har as spiders go well two actually..some native americans believe a giant spider wove the cosmos. and the other is greek
this lady aracna boasted of being the best weaver even better than helena(the goddess) helena freaks out goes down to challenge her aracna wins and helena is all like i hate you so she turns her into a spider and thats where spiders come from.
now remember in IT if I recall correctly the clown took the shape of whatever you feared most when it was the spider it was only because thats what the kid feared if he was scared of teletubbies it would have been a teletubby
except maybe ungolient none of the spiders were really evil; shelob was only fighting for survival as were the mirkwood spiders. and ungolient was fighting melkor(although i dont know why so im probably wrong about this next bit) if ungolient was evil wouldnt she have Aattacked yvanna or one of the other "good" guys?
I think spiders are just creepy and thats why theyre so prominent theyre like a universal fear.