First of all, the hurricane and the 8,000 elephants is implausible and unnecessary. You don’t need 8,000 of them to form a viable breeding population in any case.
Next, you need to factor in the fact that very few species of animal are domesticable at all or usable as riding beasts. As Jared Diamond points out in Guns Germs and Steel, so few animals can be domesticated that just a few dominate agriculture and have therefore been spread globally — the cow, goat, sheep, chicken, horse, pig, dog and cat. There are other domesticable species, including the elephant, but they prove to be less than optimal as domestic animals for a variety of reasons, explained below.
Diamond sets out six criteria that any species needs to meet to be readily domesticable:
- They can’t be picky eaters as they must be fed using what’s available around settlements.
- They must reach maturity quickly, relative to their owners’ lifespan, to make the investment in rearing them pay off. This is why Elephants, which take 15 years to reach maturity, are not more widely domesticated.
- They must be willing to breed in captivity.
- They must be docile by nature, or easily bred into docility.
- They must not panic or flee when startled, or have a flocking behaviour which enables them to be herded when they do panic.
- It’s better if they have a natural social hierarchy with strong pack leaders, so their owner can substitute for the pack leader.
Put this all together and you see why people everywhere eventually settled on the horse as the riding beast, rather than say the less-than-docile Zebra.
In your scenario, full-sized elephants might be interesting riding beasts, although the 15 years required to reach maturity would make them very expensive and long term investments. I’d love to see a dwarf pachyderm get into a Howdah or on to the back of any kind of riding beast with four tree-like legs and a trunk as a single arm/hand. Maybe carts and chariots are a better bet for your evolved pachyderms?