Though they might look like giant rats, the opossum is not a rodent. They make up the largest order of marsupials in the Western Hemisphere. Marsupials include animals like the koala and kangaroo. Only opossum females have a pouch. They have prehensile tails. They’ll eat just about anything. Interestingly, they have a remarkably robust immune system. They show partial or total immunity to the venom of rattlesnakes, cottonmouths, and other pit vipers. In addition, the opossum is eight times less likely to carry rabies than wild dogs. There are 103 (or more) opossum species in 19 genera.