Cavalry

[Brave Brave Sir Robin]

I'm actually reading a book now on the arab conquests and the author claims that most arab cavalry dismounted before fighting their Sassanid and Byzantine foes. Also a good portion of the Arab warriors did wear chain mail though their horses were unarmored. There are accounts of Persian archers slaughtering Arabs in droves at the Battle of the Bridge while at the same time being relatively ineffective at Quaysidah (sp?). So the Arabs were not necessarily as lightly armored as we might be led to believe. At least the upper classes. The Bedouin were probably unarmored.

[Vasiliyi]

Heavy cavalry can be easily countered in many different ways.

Slingers. I had 2 units of slingers mow-down 3 units of pontic bodyguards as if they were using machine guns. Honestly, the only way Cata's are good is if they are use by the human, not AI. The AI is too stupid to realize their strength.

[Maion Maroneios]

Or use Katapeltai. It is funny to see Kataphraktoi and generally every kind of Heavy Cavalry dying like flies from a shot. But then again, the mechanai are expensive, not easy to come by and clumsy on the battlefield.

But as many stated above, there are ways to counter heavy cavalry like Kataphraktoi.

Maion

[Watchman]

We may also note that around the time of the Arab conquests super-heavy cavalry was somewhat out of fashion in the Middle East. Byzantines and Sassanids alike were more into moderate armour and only partial barding (and the Sassanids apparently preferred to bow to the lance as the primary weapon, too) rather than the massively heavy juggernauts that had been all the rage a few centuries earlier.
'Course, you can kill even such "katatanks" with decent maces relatively easily enough, as the Romans' local auxiliary infantry at least once did to Palmyrene cataphracts, so eh.

Also, cavalry played a relatively minor part in the Conquest period Arab armies AFAIK. The largely arid peninsula wasn't exactly prime horse-rearing country after all.