Flying skulls

Skull bats are a type of creature within Gravity Falls, Oregon Although they haven't played a role in an episode yet, the serve as enemies in the games "Fright Night" and "Rumble's Revenge."

History
A skull bat is first seen as part of a set of promotional postcards that Dipper and Mabel Pines sent to their parents back home. Mabel's includes a photo of her holding a skull bat that has no wings, saying she found it in the woods.

Furthermore, Skull bat skulls are seen in the in various episode as an recurring background object in the Mystery Shack.

Appearance
They creatures look like a one-eyed skulls with bat wings and large fangs.

Sightings

 * Fright Night
 * Rumble's Revenge