This bug is one of the most annoying issues that i have with RTW...
The riders armour is magically transfered to the horse, because of the fact that phalanx spears are piercing and work like arrows the only defence against them is the armour of the unit fighting them and the shield this bug makes the phalanx useless against gavalary with armoured RIDERS but NOT horses, just try it yourself a companion cavalary unit with armour 10 riders will have its horses survive the FRONTAL charge as there would be cataphracts now if you reduce the RIDERS armour to 2 they dies like flies...
Now as I know the pikes hit 99% the horse and rarely the rider so if the horse had SEPARATE armour values from the rider it would not defeat the pikemen with a frontal charge...

Hellenes