Rock/Ground might not be a good defensive typing today, but it was very good in RBY. Immunity to Electric attacks meant it got a lot of opportunities to switch in and force switches. The immunity to Thunder Wave, the most reliable paralyzing move, was also very useful. The Normal resistance combined with RBY's Normal-heavy metagame and a pretty damn high Defense made Rhydon practically untouchable by Electrics. Of course, anything with a Grass or Water move would kill the beast, but Rhydon was never meant to fight those things anyway.
On the offensive, STAB Earthquake coming off a very good Attack stat was dangerous then as it is today. Coupled with Body Slam, it covered everything but Aerodactyl for neutral damage. Rock Slide was also an option, but very situational as Body Slam or Earthquake were usually better choices.