Wombat droppings are cube-shaped to keep them from rolling away from an
area they've marked as their own.

If you're interested in animal poop, then you'll find the subject of wombat
poo a fascinating one. Wombats produce cube-shaped scat which they use to
mark the area around their burrows. The wombat will leave these droppings
on fallen tree trunks or stones in order to make them more prominent. The
droppings have flat sides to stop them from rolling away.