Shape Shifter

The Shape Shifter is a an intelligent creature that Dipper Pines, Mabel Pines, Soos, and Wendy Corduroy come across while exploring the author's bunker. It can take the shape of anything that it sees.

History
The Author found a mysterious egg while excavating, curious, he nurtured and hatched it. But the creature developed quickly and became extremely dangerous. The Author imprisoned the Shapeshifter in a solid steel cage while he prepared a stasis tube to freeze the monster, but the Shapeshifter broke free of the cage, forcing The Author to flee. He sealed the bunker, hoping it will never get to the surface.

When Dipper and Wendy initially meet the Shapeshifter, it had taken the form of Baron Num Nums High Flyin' Beans' mascot and pretends to be the author of the journal. Dipper gives him the book when it says that it needs the journal to capture the a beast that escaped its cage. When Wendy and Dipper discover that he is not really the author, they run as takes the forms of various creatures in the Journal. Dipper throws his flashlight down a tunnel to trick the monster. When Dipper and Wendy find Mabel and Soos, they make a plan to launch high pressure water at him, but it hits Wendy too, causing Dipper to announce his secret to an injured Wendy.

The real Wendy then comes up behind him, hearing what he said and the Shapeshifter and Wendy wrestle each other. Wendy gives Dipper a signal(The sign that her and Dipper make when they keep a secret) so he uses Wendy's axe on the Shapeshifter. Mabel and Soos trap him in ice in a cryogenic chamber, before being fully frozen mentions that "You have no idea what you're up against." and "If you keep digging deeper, you will meet a fate, worse than you can imagine. This is the last form you will take!" The Shapeshifter transforms into Dipper screaming, then is frozen.

The Shapeshifter mocks Dipper's search for The Author, saying that the writer of the journals "isn't himself anymore", and seeing as in the previous episode the the invisible writing in the journal seems to become non-sensical and random(example: "Can't sleep! Can't sleep! Can't sleep!", "This is NONSENSE!... I don't know anymore!!!") as the journal goes on, it seems that The Author may have gone insane.

Appearance
The Shape-Shifter's normal look is a pure white slimy body with four slender legs, his right arm has a three-fingered hand, and his left arm is swollen with a clawed hand. He has pink eyes and a snout with four fangs hanging on the ouside of its mouth, and six smaller teeth on the inside.

Trivia

 * The exact origins of the Shapeshifter are unknown, found as egg, he could be extra terrestrial, native fauna to Gravity Falls, a mutation of some kind, or something else.
 * The Shapeshifter has been shown to turn into a Gremloblin, a Gnome, a Hide Behind, and five currently unknown creatures: a three-eyed toad, a creature with a fist for a head, an insectoid with an armadillo shell, a golem-like creature, and a creature that resembles a burning fire.
 * The Shape Shifter takes inspiration from John Carpenter's The Thing, many of it's adopted forms paralleling the monstruous distortions that the alien produces in the movie. However, the presence of a true form, with a rather plasma-like appearence and red eyes, reflects the description in the original novel, Who Goes There?.