Wasn't it in like ru before? Why the sudden change?
Rhyperior lends itself to various roles; it can set Stealth Rock, serve as a check to threats like Rotom-H, Volcarona, and Togekiss, and function as a wallbreaker thanks to its enormous Attack stat. Additionally, its sheer physical bulk allows it to tank hits like Close Combat from Choice Band Urshifu-S and serve as a very solid check to physically-oriented Zeraora. It is particularly good at keeping Stealth Rock up thanks to its ability to threaten common entry hazard removers in Mandibuzz, Excadrill, and Corviknight.
It seems to be quite versatile. It can set Stealth Rock and reliably keep them up by threatening top removers. It can punch holes in teams with a gargantuan Attack stat. It can check threats like Urshifu-S and Zeraora, which can be tricky to do. And as Staka~ mentioned, it can be used very well on Trick Room.
Source
Hope I helped!
When a lower Pokemon is used on 4.5% of teams on the OU ladder, it will move up to OU. This is what happened with Rhyperior, and it just barely made the cutoff at about 4.7% usage for the month of September (tier changes happen at the beginning of every month). You can see the usage stats for September here.
There are a couple reasons why Rhyperior sees usage in the tier. It has a good matchup against a lot of common threat, such as Zeraora, non Giga Drain Volcarona, and Rotom-Heat to name a few. Its massive attack stat is a really big positive for it, as shown by the rise in popularity of the Choice Band set. Swords Dance and/or Stealth Rock sets are also very viable.
