When you shoot at a character riding a monster the usual restrictions for shooting at characters apply, as explained in the section on Heroes and Wizards. However, some monsters are so big that the chance of being able to find a unit of comparably sized creatures to shelter inside or nearby is rather slim. A character riding a monster is therefore easy to pick out as a target. If the monster is large target, the enemy adds +1 to his 'to hit' score.

The normal -1 to hit that applies to man-sized characters on foot does not apply to characters who are riding steeds or monsters. The whole target is simply so obvious that there is no doubt as to where the character is! You might wish to re-read the rules for shooting at characters in the Characters section to clarify this.