Chicken Little
noun
a person who constantly warns that a calamity is imminent; a vociferous pessimist: The Chicken Littles are warning that the stock market will collapse.
Origin of Chicken Little
from a character in nursery tales (also known as Henny Penny, Chicken-Diddle, or Chicken-Licken) who, when struck on the head by an object from above, panics and believes that the sky is falling