In the case of cavalry models, all shots are worked out against the rider. If the rider is slain, the mount is removed as well. This is a convenient and practical way of representing cavalry, as it dispenses with the need for individual dismounted riders and loose mounts. Obviously some riders are dead, horses bolt and run away, and some mounts are killed, throwing their riders to the ground. But these things can be left to the imagination while models are removed as a single piece. Note that this only applies to ordinary cavalry (mounts which have 1 Wound) and not to monsters with riders. Monsters have more than 1 Wound on their profile and are covered by the rules for Monsters.