A drop bear is a carnivorous bear that is known well for hanging in the tops of trees, and dropping down on potential prey to attack, hence the name. They eat medium-large mammals, often times many times larger than itself. While there is some resemblance to a koala, drop bears have been noted to have large fangs, and be much more aggressive.
The site AustralianMuseum.net, An institute dedicated to preserving collecting and maintaining science associated with Australia, has an article on drop bears, with details on its weight, height, habits and diet. The website claims
Danger to humans and first aid Bush walkers have been known to be 'dropped on' by drop bears, resulting in injury including mainly lacerations and occasionally bites. Most attacks are considered accidental and there are no reports of incidents being fatal.
However, something that leads me to believe that the source is not valid is a comment by the user Frank Jaeger stating:
It's true. It's not even funny. A drop bear killed my kids. They attack baby goats too apparently, I take Comfort in knowing that we are at least one step closer to understanding them and if we can find out enough about them and demystify them we might one one day stand a chance...
Given, the last time this article was updated was 30 October, 2015 (11 months ago), and the comment was posted 10 months ago, it would seem important to update the article to report a fatal event.
There are many many other Articles on the internet sourcing drop bears as a vicious animal, but only 2 sources (hoaxes.net and Wikipedia), along with some skeptical users sprinkled about, refute their existence.
Do these creatures really exist?