Gremloblin

The Gremloblin  is a half gremlin, half goblin monster that lives within the Gravity Falls forest.

History
Some point subsequent to the events of the series, the author of 3 comes across and studies the creature, recording his/her observations in the previously mentioned journal.

In "Boss Mabel," Dipper Pines sets out to find an authentic supernatural attraction for the Mystery Shack, as his sister and temporary boss, encouraged him to do, and successfully brings back the Gremloblin. He releases it into a cage and shows it to a few tourists, who, upon looking into the beast's eyes, see their worst nightmares and go insane. Later, Mabel gives the monster a key to unlock his cage so he can take his five minute break, as he is "sort of an employee." The twins cower for some time as the Gremloblin thrashes the gift shop and eats their profits, 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, and fly 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, 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. 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 nightmares. So horrible they may result in the victim going insane, upon eye contact (this power can also be used on himself if he stares at a reflection of himself). He seems to possess a some level of intelligence, evidenced when he reads Mabel's note and then uses a key to escape his cage. Additionally, the beast can breathe fire, grows spikes on his back when he makes contact with water, and can grow a pairs of wings with which to fly upon his desire.