Instructions
Defeat the first 11 rivals to unlock Rival 12. The first three rivals are unlocked automatically when you start, then you must race each one in succession to unlock the next one in line.
Beat Rival 12 to unlock Rival 15. Win the races against Rivals 13 and 14 before attempting to beat Rival 15. Rival 13 only appears on Saturdays and Rival 14 only appears on Monday through Wednesday if the weather is clear.
Defeat Rival 15. Once you've bested Rival 15 in a race, Rival 16 will appear on Monday through Friday, no matter what the weather conditions are.