There are two varieties of Eye-Bats: those from Gravity Falls and the ones from the Nightmare Realm. Eye-Bats from Gravity Falls have black wings and are as large as regular bats, while Eye-Bats from the Nightmare Realm have red wings and are larger than a human. Aside from their wings and size difference, both have a yellow eyeball.
Normal Eye-Bats mostly just fly around and avoid humans. Eye-Bats from the Nightmare Realm can project a red light that freezes people into stone, and take them away.
Role in the series
They are revealed to be working for Bill Cipher and his demon pals in "Weirdmageddon", when during the event, a number of Eye-Bats turned people into stone then capture and take them to the Fearamid, the whole downtown of Gravity Falls was covered with Eye-Bats flying around the openings surveying the areas for civilians. During the final battle at least a few Eye-Bats are destroyed by the Shacktron robot.
Sometime after, the remaining Eye-Bats are chased out away by Sprott.