Very rarely, a character will have the option to ride a monstrous beast. In this case, the whole model is treated as having the troop type 'monstrous cavalry' and follows all the rules for both characters and monstrous cavalry models, even if he is riding a monstrous beast with only one Wound.

It's worth noting (as the wording is rather similar) that the rules for Ridden Monsters do not apply to monstrous cavalry mounts – they are two distinct troop types.