Wounds must be recorded separately for the rider and his mount. If the mount is slain then 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 its own side or running rampant among both armies. Remember that these big monsters are often hand-reared by their masters, and so are likely to go uncontrollably wild if their rider is slain. Roll a D6 and refer to the Monster Reaction Table to determine how the monster reacts for the rest of the game.