He is probably talking about Koalas. They are small, bear-like marsupials. They aren't really "vicious" (they usually spend their day eating and sleeping on a tree), but they can sometimes be if you threaten it or take away its food.

Your brother-in-law may also be talking about Tasmanian devils, but that is less likely since they live on an island called Tasmania which is near Australia.

Koala Bears

tasmanian devils in tasmania?

My brother in law was telling me about when he lived in Australia for a while and that he saw these miniature vicious pandas. I didn't think there were any pandas in Australia but maybe I'm wrong? And if its not a panda what else could it be, he said they were small, fast, vicious, and looked like miniature pandas?