Tachikawa-Kawasaki KI-36 fighters side by side with two Foo Fighter “bogies.”
Foo Fighters is the name given to the general body of spherical, circular, disc-like, or wedged shaped “bogies,” sometimes seeming to glow, shine, or reflect a high degree of illumination seen mostly by World War II pilots or flight crews. They usually paralleled or followed aircraft and were seen by aviators on all sides of the action, being reported by American, British, German and Japanese crews. No Foo Fighter was known or reported to have made or attempted any sort of contact, interaction or attack. They were known, however, for their high rate of speed and agility, being much faster than any known aircraft at the time as well as being extremely maneuverable, often exhibiting highly unconventional abilities such as instantaneous acceleration and de-acceleration, rapid climbing and descent, and hovering in place.
Source: Sped To Work: Foo Fighters