Arachnimorphs

Arachnimorphs, also known as "spider-people" are spider-like creatures that can take form of a Human as disguise. In the Mystery Mountain brochure, they are known to eat humans and hide among the human society and are even speculated to be the government.

Personality
Arachnimorphs are very smart and tricky, they lure their human victims with their charisma to places where they can be alone and then reveal their true forms. Darlene, the only known Arachnimorph in the series is very manipulative and tricked Stan Pines twice by just flirting with him.

Trivia

 * Arachnimorph is a portmanteau of the words "arachnid" (the class which Spiders belong in) and "morph" (a synonym for transform).
 * According to the Mystery Mountain pamphlet, if someone has sweaty palms, hairy body, ravenous appetite, and has a habit of crouching behind furniture, they are a spider-person in disguise.