You hopefully know the word 返す, which means "to return" (as in, you're doing the direct action of returning something). This word, on the other hand, is the version where something just returns (and you're not doing the action of returning). Use the る to help you to remember this. る the Kangaroo is returned to you. You don't do any of the returning, it just happens or someone else does it. That's why this word means to be returned.