Monsters are often employed as mounts for characters – an Orc Warboss could be riding a Wyvern perhaps, or an Elven Prince a mighty Dragon. If a character model is riding a monster, it does not use the rules for cavalry or monstrous cavalry, but rather those given here.
If a character has a ridden monster, the whole model is treated as having the troop type monster and thus follows all the rules for both characters and monster models. A character on a ridden monster cannot join other units.
Whilst a cavalry model is treated as a single model for the purposes of hitting and wounding, a character and his ridden monster can be attacked separately. As the battle goes on, the character may find himself unhorsed (well, un-Griffoned, or un-Dragoned anyway) or the monster might have its rider slain from off its back.