Gremloblin

The Gremloblin is a hairy half gremlin, half goblin monster that lives within 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 powers and appearance in said book.

In "Boss Mabel," Dipper Pines sets out to find an authentic supernatural attraction for the Mystery Shack, as his sister who is now the temporary boss of the Shack, encourages him to do so. After he successfully brings back the Gremloblin. He cages it and shows it to a few tourists, who claim that it is fake, but when they look into the creature's eyes, they see their worst nightmare. Later, Mabel puts a key in the monster's cage for him to unlock his cage if it ever wanted to take a five minute break, as he is "sort of an employee." When it does, the Gremloblin thrashes the gift shop and eats their profits. The twins are then forced to cower for some time before deciding to confront it. As Dipper reads in 3 that water should only be used as a last resort when fighting the creature, as it will make him stronger and scarier, Mabel rushes in and splashes a glass on him, not hearing the full sentence. She is quickly captured by the beast and, just as she looks into his eyes, her brother diverts the creature's gaze to his reflection in a mirror, causing him to see his worst nightmare, wearing glasses like his father. This terrifies the monster and it flies away from the Shack.

Appearance
True to its name, the creature has traits of both a goblin and a gremlin. He has red eyes with pupils, and they glow yellow when one stares into them. He is muscular, has green skin, clumps of body hair, and a prominent lower jaw with two large fangs protruding out of it. Mushrooms coat his shoulders and hair his arms, and his ears are pointed like a bat's. It has an immense size to begin with, and grows bigger upon contact with water, as well as sprouts spikes on his back. when in phase 2 (trigger by water), he can also grow bat-like wings.

Powers and Abilities
The Gremloblin has an assortment of unique abilities, notably the power to make others visualize their worst nightmare, which may result in the victim going insane upon eye contact (this power can also be used on himself if he stares at his reflection). He seems to possess some level of intelligence, evidenced when he reads Mabel's note and then uses a key she left to escape his cage. Additionally, the beast can breathe fire, grows spikes on his back and grow a pair of wings and becomes more menacing when he makes contact with water. His claws emit neurotoxins.