First of all, Rhydon is probably one of the worst possible gym defenders. That being said, you can do whatever you want if you want to stuff gyms with bad high CP pokemon for people to easily tear down (or alternatively for your team to be able to prestige like it's swiss cheese).
If you've already got a decent DB Dragonite, then Rhydon is basically useless on your attacking team. But if not, you can power this guy up to fight Snorlax, as Mud Slap is basically equal with Rock Smash in that particular matchup.
Otherwise, I would just keep it as a trophy perhaps. Defensive moveset doesn't really matter with Rhydon because he's a trash defender.