There are a few works that have creatures in them that form some sort of ball out of themselves and then roll around in a much more willfull and directed manner than the rolling at the mercy of gravity kind of rolling that pillbugs and, if I remember correctly, some sort of dune spider is known for who prefers to roll down a dune than walk down it. While creatures rolling themselves up into a more or less round shape is a common occurence, what I want to know is if this sort of directed rolling locomotion is possible for creatures or even a viable way to get around.

My initial suspicions of how it might work is that they have a body-length organ filled with some sort of weight that effortlessly passes from one end of the organ to the other in a repeated controlled fashion like breathing that acts as a way for it to adjust its center of gravity in a way that in its rolled-up state it will begin to roll around but I just don't know if that sort of thing is possible/plausible or if creatures rolling around as if they were bowling balls with a mind of their own will firmly remain in the realm of fantasy.