Another question from Reddit that may fit the format of this site.
Elephants used to be used for warfare in the past, and even those are hard to tame. Would it be possible that instead of horses, rhinos are used for cavalries?
Horses are used for their great acceleration and speed, as well as being able to carry quite a heavy load. They can rear up and aid the knight in battle, as well as be trained quite well to charge the enemy without shying away.
Elephants can be very aggressive when the situation warrants, and their hide is very tough and thick, making it very hard to kill. They were huge targets, however, so archers and ballistic devices could target them easily and bring them down without them so much as touching a soldier.
Rhinos, however, are a whole different beast. Sure, they can get up to some insane speeds and gore the living crap out of someone, or even a horse, but they have very poor eyesight. Not to mention, do you know anyone who would be brave enough to approach a wild Rhino? They are extremely aggressive, and would most likely be extremely hard to train. They are quite dumb beasts, and will take a few hundred years of taming and breeding in order to train them to fight.
And when they do get to the battlefield, their poor eyesight will lead them astray. They will pick up speed over time, and get ready to charge a man on his horse with a javelin, but all the horse has to do is veer off a few feet to the right when the rhino is a few dozen meters away and the rider will be perfectly safe to lop off the rider's head as the rhino charges off into the forest.
All in all, they just aren't made for that sort of thing. They are very clumsy with their poor eyesight, and would be left in the dust by the cavalries consisting of horses alone.
It might be possible to have rhino cavalries, but they would most certainly not be very advantageous.
Pros of a rhino cavalry:
Cons of a rhino cavalry
Rhinos might be used as a surprise weapon against unsuspecting armies. Then again, the strategy would be to let loose a horde of rhinos behind or at the sides of the enemy lines and send them rushing towards the targets. Using rhinos to ride into battle and use them effectively as beats of burden would be impossible considering the behavior of today's rhinos.
Not really, but it isn't what is actually meant when talking about rhinoceros cavalry either.
What it actually means is soldiers riding animals that look like rhinoceros, but actually are different in ways that allow using them as cavalry. Whether this is due to long periods of breeding them possibly with magic assist or genetic engineering, intervention by gods (paladins of the rhino god mount up!) or simply because it is a different world and the pseudo-rhinos just are different that way is up to the creator. And usually the actual details beyond the general hand wave are not relevant either.
So the question isn't really whether it is plausible, but how it is made plausible in this particular setting.
While Rhino cavalry seems a bit unlikely, it might be possible to use rhinos in a different manner.
Cavalry is traditionally used for scouting, pursuing fleeing enemy (cavalry is often held in reserve and dispatched after the enemy line is broken) or only occasionally to strike into enemy formations (especially heavy cavalry, and then only if they have ranged weapons that can outreach the enemy infantry).
The users of rhinos would be taking a herd and (carefully) transporting them into the battle area. Once in place, and possibly surrounded by a temporary corral, the army waits and looks at he disposition of the enemy. Once the key formation is identified, the rhinos are goaded into a frenzy, and then the corral gate is opened, and a very brave cavalryman on a very fast horse is waiting up front. The cavalryman rides towards the enemy formation with the herd charging right behind. With a bit of luck he can veer off at the last moment and 10-20 angry rhinos crash into the enemy shield wall/phalanx/infantry square and shatter the formation.
With a wide hole punched into the enemy line, your own forces can stream into the breech and begin the process of rolling up the enemy. Timing it right should allow you to break in while the enemy is disrupted, and hopefully they have killed or hamstrung most of the rhinos so you won't be dealing with rampaging beasts in your own formation. If you think that might be a problem, then simply remain in a defensive formation with pikes out front and wait for the enemy to collapse as rampaging beasts maul and trample their troops.
This is going to be one of those high cost/high risk/high payoff ploys that only really work the one time, so make sure you save it for a major battle, and don't spoil the surprise on a minor skirmish somewhere.
Unlike what others here mentioned, it seems that it isn't hard at all, to tame rhinos, when they don't grow up in the wilderness. And they develop a strong bond to their zookeepers.
(src, unfortunately only the German Wikipedia https://de.wikipedia.org/wiki/Panzernashorn#Interaktion_mit_anderen_Tierarten, the english one doesn't mention taming at all)
So it might be possible, to train them to be send into battle, to break up infantery formations ... but if you would be able to calm them after they went amok, I wouldn't be so sure. And if it would be worth the effort, as Rhinos grow slowly, is another question.
But for using them as cavalry, they are probably too stubborn and their (round) backs are not a nice place to be, when they stamp around.