I think we need more details on this one. Did the pony just start rearing when being mounted, or is this something has its been doing since day one of ownership? Were there any environmental factors that may have caused the money to react the way it did while on the hack? How was your friend riding at the time? How has the pony been on hacks before? Did it show any signs of bad behavior before this occured? Have you ruled out pain? Tack fit, sore back, sore mouth?
This is what happens when you have democrats in office