Drop bears are a carnivorous species of koala in Australia, believed to have come about due to mutations caused by radiation from nuclear test sites. They have developed extra-long, razor sharp claws, though otherwise remaining similar in appearance to their harmless cousins. They hunt by hanging around in trees acting like normal koala bears, until some unsuspecting prey animal is foolish enough to walk underneath. At which point they release their hold on their tree, landing on the head/shoulders of the poor animal and viciously slashing out it's throat with their claws.
Drop bears are believed to be responsible for anywhere from 80-200 deaths per year, although actual figures remain hard to come by, due to the Australian government's concerted effort to keep drop bears a secret.
Interestingly, while they are quite aggressive and seem to treat all other species of wildlife as equally attackable, they seem quite passive and friendly with normal koalas, sharing territory and never once has a koala been seen harmed by a drop bear.