First off, congratulations on that find! It's nice that it is close to lvl 30 already, so you won't have to spend nearly as much stardust/candy as you would have if it were a lower level!
If I were in your shoes, I'd save the 100% Rhyhorn for Rhyperior if/when Niantic ever releases Gen 4 Pokémon. Whether you decide to max it out now or later on, though, is up to you. But, I definitely would do it in a heartbeat!