Really? I generally don't associate the Koala with Linux-based (or anything-based) operating systems. 

Mark Shuttleworth wrote a message about the Karmic Koala. In it:

"Ladies and gentlemen, allow me to introduce the "Karmic Koala", the newest member of our alliterative menagerie.

When you are looking for inspiration beyond the looming Jaunty feature freeze, I hope you'll think of the Koala, our official mascot for Ubuntu 9.10. And if you'll bear with me for a minute I'll set the scene for what we hope to achieve in that time."

So, a Koala it is, then. Click here to read the rest of the announcement naming the Karmic Koala as the Ubuntu 9.10 mascot.