A drop bear (or dropbear) is a dangerous Australian marsupial. Drop bears are commonly said to be unusually large, vicious, carnivorous koalas (although the koala is not a bear) that inhabit treetops and attack their prey by dropping onto their heads from above. They are an example of local lore intended to frighten and confuse outsiders and amuse locals, similar to the jackalope, hoop snake, wild haggis or snipe hunt.
It is often suggested that doing ridiculous things like having forks in the hair or Vegemite or toothpaste spread behind the ears will deter the creatures.
UPDATED Decided to fix the drop bear pose for being to similar to the Nextwave comic reference.