Koala

The koala is a small, tree-dwelling, herbivorous marsupial which averages about 9kg (20lb) in weight. Its fur is thick and usually ash grey with a tinge of brown in places.
The koala gets its name from an ancient Aboriginal word meaning "no drink" because it receives over 90% of its hydration from the Eucalyptus leaves (also known as gum leaves) it eats, and only drinks when ill or times when there is not enough moisture in the leaves. ie during droughts etc.
The koala is the only mammal, other than the Greater Glider and Ringtail Possum, which can survive on a diet of eucalyptus leaves.