Riding Along with an Aggressive Mount: If your mount isn't trained for war but chooses to fight with you aboard, you can just give the mount its head. You and your mount make separate initiative rolls. Because there are some full-round actions you cannot perform while your mount moves (see Parts Three and Four), you must delay until after your mount's turn to use such actions if your mount's turn in the initiative order comes before your own turn comes. Even then your mount's movements could keep you from performing your intended action.
When it's finally your turn to act, you must succeed on a DC 10 Ride check to adjust your actions to fit your mount's uncontrolled movements. The check is a move action for you, but it does not provoke attacks of opportunity. (You're looking after yourself, not your mount.) If you fail, your mount's movements keep you from taking any actions this round. If you succeed, you can use a standard action. Just remember that you're allowing your mount to take you where it will, so your options might prove limited.