Pullbugs

Ulaan has many beasts of burden, but the strongest are gigantic round beetles with many legs called pullbugs. They look like gigantic pillbugs the size of large trucks. Dodecadromedarys are usually used for personal transport, but pullbugs are more akin to oxen with the temperament of cats. They are incredibly strong and tough, and they have the added ability to roll into a tough defensive ball when threatened. Legends say that they were once gigantic boulders that started rolling down a mountain, but instead of crashing The Sisters turned them into living creatures.
  In the wild, pullbugs roll across the dunes being pushed by the solar wind like tumbleweeds. One of their magic properties is that once they start rolling, it gets harder and harder to stop them. As long as they are well-fed and cared for, they can roll for days. Most people just attach reins and yolks to the pullbugs and use them like farm animals because they are best at crossing dirt and sand. For larger vessels pulling will not suffice in the coarse sand, so the pill bugs are trained to curl around large axles like tires on a truck. 
  The breeds of pullbugs vary from minivan sized to 18-Wheeler sized. The larger the bugs the more weight they can carry, and the larger the vessel you can create. The ships can get as large as cruise ships, and the pill bugs can travel for days at a time curled into balls.