Giant eyeball

The "" was, as the name indicates, a giant eyeball encountered by an elderly couple 30 years ago.

History
The eye somehow landed on Lake Gravity Falls 30 years ago before the events of the series, wherein he surfaced shortly afterwards and was then spotted by an elderly couple. The couple panicked, ran away and had their memories erased by the Blind Eye Society.

Appearance
The Giant Eyeball is a disembodied eyeball of huge proportions.

Sightings

 * 207. " Society of the Blind Eye"