If a character is riding a creature that causes fear or terror, then the entire combined model of rider and mount is assumed to cause fear or terror. The rider is only affected by fear and terror in the same way as the mount he is riding and any tests that are required are taken using the rider's Leadership. See the Psychology section for a detailed explanation of how this works. For example, a Dragon causes terror and so is unaffected by fear or terror - it is a huge monster and is hardly likely to be upset by smaller, less frightening creatures. A Dragon rider is also immune to fear and terror. Perched on top of a huge Dragon his courage is bolstered beyond the point where he has to worry about such things.