Hide Behind

The Hide Behind is a mysterious creature in the Gravity Falls forest.

History
Some point before the events of the series, the author of 3 comes across and studies the creature, recording its abilities in said book.

In "The Hide Behind" Dipper sets out to find the elusive creature to prove it exist. As Dipper concludes the interview with his great-uncle, the Hide-Behind makes its rattling noise and runs out from behind the Mystery Shack, into the forest. Dipper after a number of hours of chasing it, corners it behind a thin tree, which makes the rattling noise. After Dipper mistook an owl with a maraca for it he stated that the Hide-Bihnd must be a legend and nothing more, as he walks off deeper into the forest, however, the Hide-Behind extends from behind the thin tree, and makes its rattling cry, as Dipper turns around, it hides behind bigger trees, with Dipper unaware of its presence as it continues to stalk the young adventurer.

Appearance
It is very tall, slender, black, and has glowing yellow eyes.

Abilities
It is flexible enough to match the trees it's hiding behind. It has the ability to change size from the height of a tree to the size of a rock.

Trivia

 * This creature is based on the Hidebehind, a creature from American folklore.
 * Dipper once mistook an owl with a maraca for it.
 * It has never been seen by anyone in the town.