Even though most people refer to Koalas as bears, they are actually marsupials.
Picture from Stanford
This means their young are born immature and they develop further in the safety of a pouch, being carried around by their mother. In the pouch, it attaches itself to one of the two teats which swells in its mouth, preventing it from being dislodged from its source of food. The joey stays in its mother's pouch for about six or seven months, drinking only milk.