Evil spirit, 4 letters
Frightener, 4 letters
Hobgoblin, 4 letters
Mental barrier, 4 letters
Mischievous evil spirit, 4 letters
Mischievous spirit, 4 letters
Unidentified flying object, 4 letters
An unidentified (and possibly enemy) aircraft, 4 letters
An evil spirit, 4 letters
Hobgoblin, 7 letters
Imaginary threats (var.), 7 letters