A sun dog is a phenomenon in which light from the sun is reflected through ice crystals in the atmosphere, appearing to create bright spots of light in the sky.
In 1969, after having inadvertently traveled through time from the year 2267, Captain Kirk questioned whether or not the USS Enterprise, which had been detected as a UFO by an Air Force base, would simply be dismissed as a mundane phenomenon such as a sun dog or a weather balloon, as was often the case with UFO sightings of the era. (TOS: "Tomorrow is Yesterday")