Wounds must be recorded separately for the rider and his mount. If the mount is slain, the rider continues to fight on foot.
If the rider is slain, the monster must immediately take a Monster Reaction test: take a Leadership test on the monster's own Ld. If the test is passed, the monster fights on as normal. If the test is failed, roll a D6 and consult the Monster Reaction table.
If a Stand and Shoot reaction kills the mount during a charge, the model is replaced by a foot version of the rider and attempts to complete the charge normally (i.e. using his own Movement value and special rules rather than the mount's). If it kills the rider, take the test for the monster and then finish the charge if possible