A Possum (Marsupial). It has a pouch for its young like a Kangaroo.
Numerous people have reported finding possums in their toilets...
These guys can crawl up the pipes the same way a rat or squirrel can, making rodents the most likely animals to wind up in your toilets.
http://orzzzz.com/top-9-strangest-creatures-that-might-show-up-in-your-toilet.html