sorry, but you can't!
I'm pretty sure it is Lv 42 and then you evolve Rydon (evolved form of rhyhorn in case you don't know) by trading it with a protector.
Rhyhorn evolves into Rhydon at level 42 in Pokemon Emerald Version.
Rhyhorn
The evolved form of Rhyhorn is Rhydon
Rhyhorn is not available in Pokémon Black. The only options to acquiring a Rhyhorn is by trading it from Pokémon White, finding it in the Dream World and transferring it to your game or transferring one from Pokémon Diamond, Pearl, Platinum, HeartGold or SoulSilver.
you can see a rhyhorn at Pokemon league route 227 route 214 stark mountain
Rhyhorn is the Rhino Pokemon, I think.
You can catch a rhyhorn at the safari zone west of lilycove
..........level rhyhorn to 42.....get a rhyhorn from safari and evolve it to rhydon
You can find a Rhyhorn at the 3D area of the Glittering Cave.
you would have to transfer a rhyperior from diamond, pearl, platinum, heartgold or soulsilver. or you could transfer a rhyhorn or rhydon from those games and then evolve it.
You can find Rhyhorn in the Safari Zone in Fuchsia City, and you can Rhydon (which Rhyhorn evolves into) in Mewtwo's cave. Hope this helps :)