Wounds must be recorded separately for the rider and his mount. If the mount is slain the rider may continue to fight on foot if you have a separate model to represent him. If the rider is slain, the monster will behave in an erratic fashion, possibly going wild and attacking either its own side or running rampant amongst both armies. Remember that these big monsters are often hand reared by their masters, and are likely to go uncontrollably wild if their rider is slain. Take aagainst the monster's own Leadership (you may not use the Leadership value of the General for this even if he is within 12").
If successful, the player may control the monster as normal for the rest of the game. If the test is failed, roll a D6 and consult the. Note that if the monster's rider is killed in close combat, you should resolve the combat first (ie, fight the combat until one side is broken or destroyed, resolve fleeing and rallying, etc.). If the monster is victorious than make the Leadership test on the first turn when the monster would be free to move and act normally.