Pokemon Shining Diamond and Shining Pearl are seeing the return of the Sinnoh region, as do many of the Pokemon that made their debut during Generation IV. One of those Pokemon added to the national Pokedex in Gen 4 was Rhyperior, the final evolved form of Rhyhorn. So how can you get this final form? It will take a few steps, so let's get into it.
To evolve a Rhyhorn into a Rhyperior, you will obviously need Rhyhorn's evolved form: Rhydon. Level up Rhyhorn to Lv. 42, to evolve it into Rhydon. From here, you will then need to get a protector. This item can be obtained in a variety of ways, including as a reward from the Battle Tower and finding one on Route 228 in the Sinnoh region.
Related: Where to find the Protector in Pokémon Brilliant Diamond and Shining Pearl
Once you have the Protector, go to your bag and give it to Rhydon. Then, you'll need to find a trusted friend or whip out a second Nintendo Switch. Why? You will need to trade that Rhydon that contains the Protector, to turn it into a Rhyperior.
Trading in Brilliant Diamond and Shiny Pearl can be done by entering a Pokemon Center, climbing the stairs and entering the Global Room. Proceed with the trade from there, then be sure to retrieve that Rhyperior once the transaction is complete.