Gobblewonker

The Gobblewonker is a legendary creature from the cartoon series Gravity Falls. Believed to be some kind of plesiosaur descendant, this creature supposedly lives in Lake Gravity Falls, primarily around Scuttlebutt Island. The real creature has never been seen.

History
In the Gravity Falls episode "Legend of the Gobblewonker", Stan Pines takes Dipper and Mabel out to Lake Gravity Falls for the day. While there, the kids take note of the rantings of Gravity Falls' local crackpot Old Man McGucket who warns them that he saw the Gobblewonker out near Scuttlebutt Island in the centre of the lake. Dipper and Mabel ditch Grunkle Stan and join Soos on his boat in order to find the Gobblewonker and take pictures of it to prove its existence.

The twins and Soos make it to Scuttlebutt Island and they do indeed encounter what appears to be the Gobblewonker patrolling the waters surrounding the island. The creature chases after their boat, following them into a cave behind a waterfall. The monster becomes stuck in the cave mouth, giving Dipper the chance to take pictures of it. However, upon closer inspection of the trapped beast, Dipper and Mabel discover that the Gobblewonker is actually a robot built and piloted by Old Man McGucket. McGucket admits that he built the robot in order to make the townsfolk pay attention to him.

At the end of the episode, Dipper tosses his camera into the lake where it is shown to be eaten by the real Gobblewonker.

Appearance
The Gobblewonker is a large aquatic creature similar to the ancient plesiosaur. It has a bulbous body with a long neck, tail, and four short fins on its body to propel it through the water. Its scales are a dull aquatic green and it has long fangs jutting up from its lower jaw. It also has a mane of sorts running down the back of its neck and its back.

Trivia

 * The Gobblewonker was clearly inspired by real-world urban legends of other aquatic cryptids such as the Ogopogo and the Loch Ness Monster.