Baby oppossums are born underdeveloped, since they are marsupials. You wouldn't have been able to recognize it as a baby anything if it was a possum. It was probably newborn when you found it because it was still alive. Let it lap up the milk that goes on its lips. I know with rabbits they can't really suck at the bottle, they lap up the milk as it comes out, so maybe its the same with this guy.