arachne
A common variety of monstrous spider girl in which the human parts beneath the waist/torso have been entirely replaced by the body of a spider. Extra eyes are a common addition. Good examples can be found in Rachnera Arachnera's character sheet or the Monster Girl Encyclopedia entry.
Note that merely having spider parts (e.g. post #1282030) is not enough for this tag. For spider-legged girls retaining human legs, consider arthropod limbs and extra legs.
For monster girls of similar body type but not based on spiders, see centauroid.