Obviously a large monster is less likely to suffer from fear or terror itself. There is no way a huge Dragon is going to be scared of a Troll, for example. These special liabilities also apply to any rider of a large monster too, so a Dragon rider wouldn't be afraid of a creature that would frighten him were he on foot.
The following rules apply:
A creature that causes fear is not affected by enemies that cause fear. Faced with an enemy that causes terror, a fear-causing monster only suffers fear, not terror. For example, a Troll causes fear and a Dragon causes terror. The Dragon is not at all worried by the Troll, but the Troll fears the Dragon.
A creature that causes terror is not affected by fear or terror at all. For example, a Dragon rider is completely unworried by fear or terror because his mount causes terror.
It sometimes happens that an enemy unit of ordinary troops is led by a mighty hero or a monster which causes fear or terror. In this situation you must test for fear/terror if a charge will result in you fighting the hero or monster in question.
In the case of terror you must also test if you are within 8" of the creature at the start of your turn, but not necessarily because you are within 8" of the unit as a whole. However, if you are charging a unit of troops in the side or rear, so that you won't have to fight a monster in the front rank, then you do not have to test. Basically this is common sense - if you don't have to confront the beast then no test is required.