Here's how to find Rhyhorn:
1. Obtain the fourth badge: Defeat Clair at the Blackthorn City Gym.
2. Head to Route 36: This route is located to the east of Blackthorn City.
3. Explore the grass patches: Rhyhorn is found in the grass patches on Route 36. It's a relatively common encounter, but it may take a few tries to find one.
Tips for catching Rhyhorn:
* Use a Poké Ball with a high catch rate: Ultra Balls and Great Balls are recommended.
* Lower Rhyhorn's HP: Weaken Rhyhorn before attempting to catch it to increase your chances of success.
* Use a status effect: Paralyze or sleep Rhyhorn to further improve your catch rate.
Good luck finding your Rhyhorn!