Flying Brain was an interdimensional demon species. They had the appearance of human brains with thin tentacles, and the abilities to fly, create force fields resistant to bullets, and mind manipulation. These demons also appeared to have glowing blue eyes during mind control, while the controlled individual's eyes would have the same glow.
Through the Restless Door, the Mistress, Soul Glutton, and Sculptor released a group of Flying Brains in the state of Montana, controlling various civilians against the local army. Willow Rosenberg was sent there and was able to disarm their force field, allowing them to be killed by the army's guns.