Thank you for your imput! However:
Despite what most people may think, Orcs are not supid. They may migrate more to violence to solve their problems, be rash, stubborn and thickheaded, but Orcs have the capability to be as smart as any other member of any other race. The mindset of most Orcs, however, prevent attributes such as what some may consider 'smart' to be shown. Yes, maybe Orcs cannot write great novels or make brilliant inventions as the humans can, create amazing architextural feats like the dwarves, or be as knowledgable in the ways of the forests as the elves, but they are in fact intelligent in their own ways. They can be described as wise, strategic, and show a great promise to be able to learn. Learning, not the ability to show what you have learnt, is what I believe is the true definition of intelligence.
I fear I have gone off topic, however, so I will summerize.
Braduk was a wise and inventive chieftain, making many intelligent decisions for his clan despite his need for vengance and retribution. I belive that it is indeed possible Braduk would have thought that the rhinos could be used for mounts, and indeed perhaps with the help of the Goblins in kenuk, he could have come up with the idea of selective breeding.
The reason I made the Braduk Rhinos become so big so quickly was due to previous lore being written involving the rhinos. Already I have extended the time by one hunded years compared to the previous lore, and by doing so have created conflict in already existing lore involving the Braduks. By making the time of selective breeding smaller, I had hoped to prevent further clashes with lore on the Braduks. I will, however, at your request, change the time that it took for the rhinos to grow to their current size.
Additionally- About the Rhinos being available to other races. Currently, the only Braduk rhinos in Anthos are in the possession of the Braduk clan, the original beasts being in a land far away; Braduk Rhinos only existing here due to migration with the Braduk clan. If the Braduk clan was to fall however, or a few are stolen, yes, it is possible for other races to breed and have access to the Braduk Rhinos. But- I would much rather this wouldn't happen. Braduk Rhinos are supposed to be restricted beasts, being hard to breed, tame and control, even for the Braduks. Having every third person (hyperbole) riding around on giant rhinos isn't exactly what I would like to release upon Anthos, and such I have made the animals hard to spread and more of a nuisance to breed than what they are worth. I plan on making a thread in the Orcish Badlands Roleplay section where people can apply for a Braduk Rhino, given enough sceenshots of training and such, just to control how many people can have them. It will be a system close to what is used in Magic, and each member will be carefully watched by myself and other responsible people.
Thank you for your questions, and your contribution to the Braduk Rhino lore.