Our dog Pepper went outside in the yard to pee and there was a possum digging in the mulch beds. Pepper started running and possum started running. The possum moved fast and made it over the fence before Pepper caught it.
This is just like what happens with squirrels, but they are smaller. The possum was bigger than our small dog. I don't know know would have happened if the possum held its ground and hissed at Pepper. Just like one did to me one time.