You have to give Rhydon a protector item and trade it. When the trade is complete, it will evolve. (Make sure you have a trustworthy freind who also has the game so he/she can trade it back to you.) By the way, it evolves into a Rhyperior.
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Basically Rhydon doesn't evolve on a certain level. You need to have Rhydon hold a Protector and Trade it, with a another person.
Rhydon, in Pokemon FR/LG, does not evolve.
Rhydon cannot evolve in Pokemon Silver/Gold/Crystal.
..........level rhyhorn to 42.....get a rhyhorn from safari and evolve it to rhydon
Trade it with the Protector on it