Number 187 in the Sinnoh Pokedex is Rhydon. Eolves into Rhyperior, and its pre-evoloution is Rhyhorn. In order to obtain Rhydon, you must go into Victory Road. He is very rare on the 1st floor. Other than that, you must trade with another game. Platinum can trade with Diamond, Pearl, Heartgold and Soulsilver. That's all i know, it may be able to trade with even earlier generations. I hope this helped you :D