Flying skulls are a type of creatures within Gravity Falls, Oregon, which frequently appear as enemies in the games.
A flying skull is first seen as part of a set of promotional postcards that Dipper and Mabel sent to their parents back home. Mabel's includes a photo that she is holding a flying skull that has no wings, saying she found it in the woods. Flying skulls are seen in various episodes as a recurring background object in the Mystery Shack.
The creatures appear as skulls with one glowing eye, bat-like wings, and large fangs with no lower jaw.