Arachnimorphs, also known as "spider-people," are spider-like creatures that can take the form of a human as a disguise. The Mystery Mountain brochure says that they are known to feed on humans and hide among the human society and are even speculated to have infiltrated the government.
In "Roadside Attraction," as Stan, Mabel, Dipper, Soos, Candy, and Grenda approach Mystery Mountain, Candy reads a pamphlet about Arachnimorphs. Later, Stan meets Darlene, whom he assumes to be a human. She leads him to Widow's Peak, where she reveals herself to be an Arachnimorph and ties him in a web. After Stan calls for help and is freed by the kids, Darlene chases them. Stan and the kids then board the Trambience, which is too slow to escape Darlene. Once Darlene is on top of Stan and the kids' car, Candy drops the car onto a geyser, which propels them to the statue of Paul Bunyan. The statue breaks and traps Darlene under it.
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.
When arachnimorphs lure their victims, they take on the general appearance of a normal human. Darlene, the only arachnimorph seen, takes on the appearance of an attractive woman when luring her prey, and the "Spiders of Oregon" pamphlet shows a male version of an arachnimorph. In their true form, Arachnimorphs have large spider abdomens and spider legs with their upper body either being their human disguise or a spider's body which they can switch between.
- 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, a hairy body, a ravenous appetite, and a habit of crouching behind furniture, they are a spider-person in disguise.
- The name 'Arachnimorphs' could be a reference to the book series and TV show Animorphs. The main characters are gifted the ability to turn into various animals for short periods of time.
- An Arachnimorph silhouette is seen on a page in Journal 3.