As a single model, the monster and its rider are considered to be a single target. It is not possible to shoot specifically at either the rider or the mount.
When you shoot at a character riding a monster, the rules for shooting at characters apply, as explained in the section on Characters on page 74. However, if the monster is a large target, the enemy adds +1 to his To Hit score.
Once you have established how many hits have been scored, you must apportion them between the rider and the monster. For each hit scored roll a D6; on a roll of 1-4 the monster has been hit. On a 5 or 6 the rider has been hit. Roll to wound the monster or rider as normal.
Work out wounds separately on the rider and his mount. Take any saving throws due to the target as normal. Most monsters do not have an armour saving throw as they have no armour, but some have scaly skin that confers an equivalent save.
Riders are permitted saves for their armour, but remember that they do not receive the additional +1 armour save as cavalry models do.
If a monster has two or more riders (a very unusual combination) then randomise any hits among the two riders. If models are glued in place (as is likely) it will be necessary to make a note of any casualties suffered.
If you use a weapon or spell that uses a template against a ridden monster, all the riders and the mount are automatically hit if the monster's base is entirely covered by the template, and all can be hit on a 4+ if the monster's base is only partially covered (roll separately for each). If the hole in the centre of the round template is on the monster's base, randomise if the rider or the mount is hit by the higher Strength hit as you would for a hit from normal shooting.