Despite the wide variety of movement strategies employed by Earthling animals, up to an including curling their bodies into circles and rolling, there is one mode of locomotion that is glaringly missing: nothing slinkies!
What I have in mind here is more a whole class of creatures than one specific creature, but to keep it from getting too broad, let's focus on the base case of a simple slinky worm:
The basic body plan is a simple muscular tube, externally symmetric, capable of stretching out or squishing down (like any soft worm) and capable of balancing upright on either end, such that it can move by flipping end over end like a slinky: with one "foot" planted, the other foot is lifted, flipped over the top, set down on the other side, and the process repeats.
So, how / why might such a thing evolve? As lemmas to that overarching question, where is its mouth / anus (does it run from one foot to the other, does it have a "head" in the middle of the body)? What kind of sensory organs does it have? Maybe eyes on stabilizing stalks in the middle, like a squibbon, or eyes ringing the feet? Aside from conforming the the basic end-over-end tube plan, anything goes.
A list of all Anatomically Correct questions can be found here: Anatomically Correct series.