Bred to carry a Ryder, their naturally segmented shell/vertebrae are protection for the animal from predators. The larger shell over the main organs has over the centuries been utilized by the native sentients to provide a saddle to make riding the Beists easier. They shape the saddle as it is forming while the Ryderbeist is still developing and its shell is still soft.
They're very alert, and can see and hear things at great distances.
Ryders, like many other creatures, sentient or not, still maintain some outer skeletal displays on various parts of their bodies. In this case, the temples.
Though the cacti look a lot different from each other, I tried to maintain some similarities to make it appear they at one time may have had similar ancestors.
When designing an alien world, it's important to think about the processes of evolution and the influences of environment on those processes.
Relating them to familiar traits on earth help the viewer relate to the subjects in the painting and allows them to read in their own interpretations of the story your painting tells.