So in Platinum, I newly taught my Monferno Hidden Power... Water. Then I evolved it and now its showing my Hidden Power Ground. I haven't used any hack. No cheats. What happened?
Are you using a ROM? Because Hidden Power type is based soley on IVs, and those dont change through evolution. If you are using a ROM there might be a glitch. And are you 100% sure it was water? Anyway, there was probably a glitch in your game if this happened.
Hidden Power is based on IVs. Did you lower your Monferno's IVs or someting. For further explanation visit: http://bulbapedia.bulbagarden.net/wiki/Hidden_Power_calculation