I'm going to get this out of the way now:
"A counter must be able to both switch in with little to no risk to itself and pose an immediate threat to the pokemon it's supposed to counter."
The reason this thing scares me so much is that it has 140 base attack, which by definition makes it a fine candidate for Choice Band. If "nothing counters CB Heracross", you could pretty much say the same thing about Rhyperior.
Adamant, 416 Attack Rhyperior's Choice Banded Earthquake/Stone Edge on a 384HP/350Def (max/max) Slowbro: 50-59%
Adamant, 416 Attack Rhyperior's Choice Banded Megahorn on a 384HP/350Def (max/max) Slowbro: 80-94%
Adamant, 416 Attack Rhyperior's Choice Banded Earthquake on a 401HP/297Def (near-max/near-max) Swampert: 57-67%
Adamant, 416 Attack Rhyperior's Choice Banded Earthquake/Stone Edge on a 464HP/240Def (max/max) Vaporeon: 60-71%
When you consider that Rhyperior gets to 179 speed and can (and should if it's Choice Banded, in my opinion) outspeed all of those proposed "counters", you can start to see how few geniune counters there really are to this thing. Especially since Vaporeon and Slowbro's spreads leave for absolutely no SpA, which both need a bit of to OHKO min-HP/min-SpD Garchomp (Vappy needs 40 Spa EVs, Slowbro needs 120).
Now for stuff that outspeeds it:
Adamant, 416 Attack Rhyperior's Choice Banded Earthquake/Stone Edge on a 304HP/206Def (~66%/min) Starmie: 108-126%
Adamant, 416 Attack Rhyperior's Choice Banded Megahorn on a 304HP/206Def (~66%/min) Starmie: 172-202%
Starmie's speed is its best defense against Infernape, hence the inability to add any actual Defense EVs when you also consider Infernape threatens a Schemed Grass Rope as well.
Adamant, 416 Attack Rhyperior's Choice Banded Earthquake/Stone Edge on a 384HP/286Def: (near-max/near-max) Milotic: 64-75%
More manageable I suppose, but pretty much only because Milotic has Recover.
Adamant, 416 Attack Rhyperior's Choice Banded Earthquake/Stone Edge on a 401HP/361Def (near-max/max) Suicune: 47-55%
Looks good on paper, but this leaves Suicune with like 219 SpA, which will never OHKO Garchomp, so this spread probably isn't nearly as realistic (or, if so, reliable) as it was in Advance.
Adamant, 416 Attack Rhyperior's Choice Banded Megahorn on a 444HP/372Def Cresselia: 65-77%
Adamant, 416 Attack Rhyperior's Choice Banded Stone Edge on a 444HP/372Def Cresselia: 41-48%
Probably the best counter since it's guaranteed to be faster at minimum 206 Speed, is immune to EQ, has the option of Moonlight even in spite of a purported Sandstream-heavy metagame, and has a Grass Rope that will nearly OHKO Rhy's heavy ass even from its minimum 186 SpA.
Adamant, 416 Attack Rhyperior's Choice Banded Stone Edge on an Impish 364HP/284Def (max/max) Flygon: 32-37%
Adamant, 416 Attack Rhyperior's Choice Banded Megahorn on an Impish 364HP/284Def (max/max) Flygon: 50-59%
Might be a better counter than Gliscor since both boast Feather Rest and an Earthquake immunity but Flygon has better base attack (100 vs 95).
Adamant, 416 Attack Rhyperior's Choice Banded Stone Edge on a 320HP/372Def (near-max) Weezing: 57-67%
Adamant, 416 Attack Rhyperior's Choice Banded Megahorn on a 320HP/372Def Weezing: 23-27%
Of mention because Weezing can realistically boast HP Ice for Garchomp already since it has an oft-overlooked 85 base Special Attack...you can even go HP Grass for a guaranteed 2HKO.
For completeness, here is what pokemon that matter can do to Rhyperior:
Jolly, 254 Attack Dugtrio's Choice Banded Earthquake on a 372HP/296Def (4EVs/min) CB Rhyperior: 50-59%
Jolly, 254 Attack Dugtrio's Life Orbed Earthquake on a 372HP/296Def (4EVs/min) CB Rhyperior: 44-51%
Revenge-killing at its best for Duggy unless it goes Adamant.
Bold, 220 SpA Weezing's Hidden Power Ice on a 372HP/146SpD (4EVs/min) CB Rhyperior: 27-32%
Bold, 220 SpA Weezing's Hidden Power Grass on a 372HP/146SpD (4EVs/min) CB Rhyperior: 55-65%
Would probably just go HP Ice for Garchomp since Grass doesn't OHKO and TB/FT/HP Ice/Pain Split is a fantastic Gyarados/Heracross/Garchomp counter all-in-one and you can Pain Split Rhyperior or whatever switches in to replace it.
Bold, 186 SpA Cresselia's Grass Rope on a 372HP/146SpD (4EVs/min) CB Rhyperior: 79-93%
May not be worth it to give it many SpA EVs since this is a 4HP EV Rhy and people may choose to give theirs max HP.
Jolly, 339 Attack Weavile's Choice Banded Ice Punch on a 372HP/296Def (4EVs/min) CB Rhyperior: 50-59%
Jolly, 339 Attack Weavile's Choice Banded Pursuit on a 372HP/296Def (4EVs/min), switching CB Rhyperior: 40-47%
Jolly, 339 Attack Weavile's Choice Banded Pursuit on a 372HP/296Def (4EVs/min) CB Rhyperior: 20-24%
So Weavile can't really pose a threat heads up against Rhyperior until it's at around 50% HP (and it obviously can't switch in).
Adamant, 405 Attack Tyranitar's Choice Banded Earthquake on a 372HP/296Def (4EVs/min) CB Rhyperior: 52-62%
Adamant, 405 Attack Tyranitar's Choice Banded Pursuit on a 372HP/296Def (4EVs/min), switching CB Rhyperior: 48-56%
Adamant, 405 Attack Tyranitar's Choice Banded Pursuit on a 372HP/296Def (4EVs/min) CB Rhyperior: 24-28%
Pretty much the same mindgame deal as with Weavile.
To wrap up, Skarmory isn't a Rhyperior counter cause it doesn't really pose a threat outside of Steel Wing, and Tangrowth is kinda iffy like Slowbro is thanks to its low initial speed. This is just the CB variant too, which poses enough of a threat in itself to make you forget about the Swords Dance one.
I've probably covered everything that's a considered switch into Rhyperior, even stuff that shouldn't be like Starmie and even Suicune to an extent. If this thing got Power Whip for some stupid reason it would be so broken lol
Oh and ADH don't bother editing your post with my calculations, people should really take the effort of scrolling down a little bit to read the thread!