Taking into account the answer by akaioi to my previous question (Would a human-like symbiotic pair of the same race be possible?).
Is it okay if we turn it around a bit?
That is, have the Tank be the youngster, and the Jockey be the older one. Why? Well ... it makes more sense for the old one to be the clever one. Imagine a species which evolves in an environment poor in food except for certain periodic windfalls, like a salmon run which only comes every several years. Here's how it works:
The adult, stable form of the creature is the skinny one. It needs all its cunning to survive the lean years, and its smaller form requires less food. Shortly before the salmon come, the creature spawns. The younglings gorge on the plentiful food, and grow large, fast. They'll need all this extra mass, because they're not yet smart enough to hunt the other, more clever prey in the region. When the lean season comes, the youngsters go through a kind of "reverse molting" where, as they deplete their food reserves, they shed their outer layers to reveal a smaller armor shell. Eventually you're left with the adult-form creature.
So there are actually four forms, of which we only care about two:
Newborns — too small to be ridden, they're busy eating salmon and getting fat
Tanks — These are the adolescents; they're big, dumb, and just the right size to bear a Jockey
Jockeys — These are the dangerous ones. Still vigorous, smart, nasty, and fast.
Geezers — Jockeys eventually get too old to ride into battle. So they retire, sit on the porch, and tell lies to the wide-eyed Tanks
I like this as it could help for the development of their civilisation, when the period of large amounts of food come it’s possible for the tribe/group to go to battle with others in an attempt to gain an advantage or surplus for the harder times to come, so all the tribes will do this and then return to their homelands and as the food runs out the warring season will end with few attempts on others resources apart from maybe the more cunning of geezers/ jockeys.
Also with the old being smarter trades and such could develop as knowledge can be passed on so the elder geezers would be the most knowledgeable fighters and crafters using the brutes for manual labour or even guiding a jockey-brute pair to teach them. The geezers could even become unsuitable by for jockeying having another quick growth spurt one good season and then deteriorating from there so that they are strong enough for a while to carry out trades and be also be leaders. (https://i.stack.imgur.com/tAGwe.jpg)
My question for development on this idea is what kind of large food event could take place in the arid desert conditions that I described, somewhat similar to the deserts of Mexico.
Maybe their staple diet for the low periods could be a cactus type plant surviving on the heavy rainfall of the periodic event?
Another thing is what kind of culture may develop for this race due to what I have described, especially with the warring seasons at the end of the food surplus.
Thanks to akaioi for the great idea that I’ve chosen to run with.