In my post-apocalyptic world, rhinos are used as beasts of burden. However, in order to survive harsh conditions and harsher predators, they need to have some sort of armor (natural or unnatural). However, I'm not quite sure how that would work. Should they have shells? How should these rhinos be protected?
Hanna-Barbera had the exact same problem for their series the Herculoids.
They addressed that need with Tundro, the up-armored rhino that shoots plasma balls from his forehead horn, because the best defense is a good offense. A brute of a cargo carrier, mega-traction with its mutant-like ten legs, and the heart of a warrior, always ready to protect his adoptive family -- The Herculoids.
Rhinos are already armored powerhouses. Most small arms would do little more than anger them. In a post apocalyptic world, the weapons needed to bring them down without risking a good deal of loss of life would be rare. I'd recommend vanilla rhinos or just slightly toughened hides (rendering them all but immune to small arms fire).
Make story about rhinos a bit longer. They could survive far away from human's settlements. Then, species started to grow up their population at railway speed, because of the human absence. With growing population their habitat also was spreading out, while some day it reached humanity. The only question is how they survived from predator, but this is wild nature, they would survive and reproduce while they have good environment for. Human settlements around with poachers and hunters can break the balance.