When UFOs appear in the vicinity of animals they usually exhibit signs of extreme fear. Pet owners and farmers have reported cats yowling and fighting, horses whinnying and kicking in their stalls, ducks quacking, cows jumping and temporarily ceasing to give milk. Dogs have whined, barked, pricked up their ears, crawled in abject fear and curled up in tight balls under furniture. Occasionally, their fur stands on end. In some cases, frightened dogs have developed illnesses immediately after the sightings, death following within several weeks. Owners have made a direct correlation between such deaths and the UFO sightings which preceded them although there is no proof to support such a contention.

UFOlogist have conjectured that some UFOs emit a high pitched sound beyond the range of human hearing which is distressing to animals. Others have hypothesized that UFOs produce a stimulus outside the realm of the human sensory range but detectable by the legendary sixth sense attributed to animals.