CONTAINS SPOILERS
Horse Whispering
If you get one of your miis to LVL15 friendship with your horse, you can unlock the "Horse Whispering" skill. Think of the skill as a "final attack." The effects differ between classes, but have powerful impacts. However, do note that only one mii can be a "horse whisperer." Any other character with LVL15 horse friendship will not receive it.
