From Discworld & Terry Pratchett Wiki
The drop bear is a predatory distant family member of the Koala which drops from a tree onto its prey. As a result the species has developed well padded backsides to cope with the drop. It lives in XXXX, although many doubt they exist at all. Rincewind was pretty sure he was attacked by them in The Last Continent. The drop-bear, or at least one of them, learnt a fairly important evolutionary lesson: never drop bottom-first onto any target, however tempting, which is wearing a sturdy and rigid pointy hat on its head. Whether it is capable of having any descendants to pass this lesson on to is a different story.