Beasts, Giant spiders
—From Codex entry: Giant Spider
Spiders, also called "deep crawlers" by the dwarves, grew in the depths of the dwarven Deep Roads, once having been encouraged to multiply to feed on the numerous species of large bats that the dwarves considered pests. Once the Deep Roads were lost to the darkspawn, these spiders began to feed on genlocks as well as bats, and their numbers were no longer controlled. Some moved up to make their lairs in the surface forests, but most have remained below ground.
The party members should support each other whenever spiders arrive. If you end up apart, a spider's web or Overwhelm attack can incapacitate a lone companion. When webbed, the companions who aren't trapped should defend their comrade. Against an Overwhelm attack, everyone else should immediately focus on the charging spider to kill it before it kills your companion.
One spot where you can find nearly all of the spider types is Ortan Thaig.