Hidden Power's power and type vary depending on the individual Pokemon using it, ranging between 30 and 70 power. There is no way to change a Pokemon's Hidden Power type or power, so you are stuck with what you have. Hidden Power is always a Special-based move.
Ruby: GOLDEEN is a very beautiful POKéMON
with fins that billow elegantly in water.
However, don’t let your guard downaround this POKéMON - it could ram you
powerfully with its horn.
Sapphire: GOLDEEN loves swimming wild and free
in rivers and ponds.
If one of these POKéMON is placed in anaquarium, it will shatter even the
thickest glass with one ram of its horn
and make its escape.