I think in the first three generations of the entire series, the most difficult one to capture is Rhyhorn. The quickest way is that you can choose to buy 6IV Shiny Ditto to get one.
In the first generation, you can only find it in the Safari Zone or Cerulean Cave. It was exclusive to the second generation Victory Road. In the third generation, you are restricted to the Safari Zone, trading or breeding a Rhydon.
In Pokemon Sword and Shield, Rhyhorn is a Ground and Rock Type Spikes Pokemon, which makes it weak against Ice, Fighting, Ground, Steel, Water, Grass-type moves. You can find and catch Rhyhorn in Giant's Cap with a 30% chance to appear during Normal Weather weather.
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