What about solid rock? and much better defenses?
Rhydon's defenses are 105/120/45, and Rhyperior's are 115/130/55. It's got 10 extra points in each of its defensive stats.
Rhydon and Rhyperior have exactly the same Speed, and Rhyperior has an extra 10 points in Attack.
Rhyperior has Solid Rock, whilst Rhydon takes no recoil from Double-Edge.
Obviously, Rhyperior is better than Rhydon, but is it THAT much better?
What can Rhyperior OHKO that Rhydon can't? Not much, I suspect. There is nothing Rhyperior outspeeds that Rhydon can't.
Defensively, this is a different story. Rhyperior's ability, coupled with its slightly higher defense stats can sometimes allow Rhyperior to stay in a turn longer than Rhydon, but, again, sometimes, it doesn't make a difference.
Last month, Rhydon got 911 usages in UU. It ranked 108th.
Rhyperior does have advantages, but are they really so significant that Rhyperior can make, say, the top 10? Will Rhyperior's Solid Rock and its slightly higher defensive stats and Attack get it an extra 8,000 usages a month than Rhydon is getting?
I can't see it happening. If it does happen, Rhydon is being underestimated imo.