How do you catch rhyperior on Pokemon Diamond?

In Pokémon Diamond you cannot catch Rhyperior however you can find wild Rhyhorn for capture in Route 227 and in Stark Mountain and then you can evolve it into Rhydon at Level 42 and then you can attach the Protector item to Rhydon and trade the Rhydon over to another game and it will evolve into Rhyperior additionally you can catch wild Rhyhorn in Routes 227, 228 and in Stark Mountain and then you can trade it while it has the Protector item attached in order to evolve it into Rhyperior.

You can't exactly catch Rhyperior- you have to trade a Rhydon holding a Protector and he'll evolve after the trade...