Complete the game and return to the starting area (Blackfoot Territory). From the LEFT of the HIGHER boulder leading up to a ledge and gap, jump and spur your horse so it lands on top, then onto the ledge. Cross the next two areas. Stand between the only two rocks in the water. Press and hold L1+L2+R1+R2+X for a few seconds until you hear a strange sound. Go to Options (press START),for a new selection - GUN. Choose any Mission or Cutscene. You will retain ALL weapons, items, and skills.
Successfully complete Story mode to unlock Magruder's seven barrel Nock gun. Successfully complete Story mode and all side missions (100% completion) to unlock Reed's armored horse and Magruder's Cannon Nock gun. This horse cannot be killed and if ever lost, it will return to the mine. This horse also does not lose any health from sprinting.