You can either go to victory road and catch it there or evolve a rhyhorn at lvl 42
Hope this helps :)
Rhydon evolves when the "Protector" is held and the Pokemon is traded.
At M.T Stark
Trade it with the Protector on it
You have to get the Moon stone.
to evolve rhydon you need a protector. I think you can get one on stark mountain. Then you have to give it to Rhydon and trade with a friend.
you can find one in victory road.
You cannot find rhypierier on Pokemon Platinum. You have to trade rhydon while it's holding the protector.
The evolved form of Rhyhorn is Rhydon
Trade Rhydon while it holds this to get a Rhyperior
you give rhydon the protector then you trade him to a friend then get the friend to trade him back and you have your self a rhyperior
When looking at the item, it does say "a Pokemon loves this item". It is a hint to give a Pokemon that item. Well that Pokemon is a rhydon. Give a rhydon that item to hold. Trade it and see what happens...
You can encounter a Rhydon on Victory Road on the first floor. A trainer on victory road will also have a Rhydon in his party.