In the U.S., at least the southern part, the terms Possum and Opossum are interchangeable. We only have the opossum here. The pictured Possum is in Australia I believe. Also, while they do indeed eat ticks, etc. and are pretty okay animals to have in your yard, they do often enjoy eating chickens and eggs. I've seen them do it. My dog keeps them mostly away, though one does get through now and then. My cats seem to totally ignore them. Of course they are nearly the same size.
Ditto for the Rat Snakes! Theoretically they are good to have, they will eat rodents and take up a niche that might otherwise be filled by more venomous snakes. However, when allowed a choice they seem to prefer young chickens and eggs. I have literally seen them ignore mice in favor of eggs! That is why in the U.S. South they are routinely called "Chicken Snakes", which irritates the herpetologists. I routinely catch and relocate the snakes early in the season, but later on I get more final. We don't get enough eggs to feed us AND the wildlife!