Monsters are often employed as mounts for characters. A monster and its rider or riders count as a single model in the same way as a cavalry model, although different rules apply.
Horses, wolves, war boars and other similar sized creatures that only have 1 Wound are covered by the rules already described for cavalry. This system is fine for these smaller creatures, but obviously wouldn't work for big monsters such as Dragons, which are far larger and much more difficult to kill than a horse or a wolf.
If a mount has 2 or more Wounds, it is classed as a monstrous mount and the following rules are used. These rules would therefore apply to a hero riding a Griffon, a Wizard mounted on a Wyvern, a Dragon and its lordly rider, and so on. No additional +1 is added to the rider's armour save, as the advantages of riding the monster are worked out in other ways instead.