Used to Elephant means a special training, for example Macedonian successors put in the same stable Elephants and Horses to get acustomed with smells, and they used trumpets for the sounds. This is very specific. In Zama, on Carthaginian side, Elephants were young so not well-trained to battle and easy to panic. The simple fact that you have Elephants in an army list do not give an automatic "used to Elephant" to the cavalry. And if I rememdered, at Zama, Elephants were put in front line in a very basic strategy... they charged, romans were not in their usual formation and let spaces between manipules to let velites harrassed Elephants who were also panicked by the roman trumpeters. Cavalry fought on each sides of the army but not at the centre.