Rhydon is actually a pretty nice support Pokemon that can sponge a lot ot things. C rank could be fine for it, but i wouldn't mind it being a little bit lower. Rhydon's EdgeQuake coverage has a pretty useful purpose, while being able to switch into moves like Thunderbolt (lol Lightning Rod Rhydon is actually useful) or the so common fire type moves. However, it's typing bugs me a bit. For an offensive Pokemon, Ground/Rock is an amazing combination of STABs due to the legendary EdgeQuake coverage, but when it comes to defensive, it's just not a good idea. Like, you're weak to Water, Fighting, Steel, Grass, Ice and Ground, and all of these types have strong representation in the VGC metagame, like Aegislash for Steel; Milotic for Water; Amoonguss for Grass; Terrakion, Scrafty, and Conkeldurr for Fighting, any bulky water that isn't called Rotom-W for ice, and Landorus-T for Ground. Having bad matchups against these common Pokemon makes its niche not as good as it should be, and in the end of the day, you'll probably end up using Rhyperior. It's just my opinion about how i felt Rhydon during the 1-2 days i played with it.
Synergy with Char-Y is important when talking about Rhydon, but putting that aside for a moment:
252 Atk Terrakion Close Combat vs. 0 HP / 0 Def Eviolite Rhydon: 116-140 (64.4 - 77.7%) -- guaranteed 2HKO
0 Atk Rhydon Drill Run vs. -1 4 HP / 0 Def Terrakion: 186-222 (111.3 - 132.9%) -- guaranteed OHKO
0 Atk Rhydon Earthquake vs. -1 4 HP / 0 Def Terrakion: 174-206 (104.1 - 123.3%) -- guaranteed OHKO
Yes. A Bashful, completely untrained Rhydon beats Terrakion. Add things like natures and EVs into the equation and it's just a bother at best.
252+ Atk Conkeldurr Drain Punch vs. 36 HP / 12 Def Eviolite Rhydon: 84-102 (45.4 - 55.1%) -- 45.3% chance to 2HKO
204+ Atk Rhydon Drill Run vs. 248 HP / 0 Def Conkeldurr: 76-91 (36 - 43.1%) -- guaranteed 3HKO
Yes, Conk wins.
252+ Atk Scrafty Drain Punch vs. 36 HP / 12 Def Eviolite Rhydon: 66-78 (35.6 - 42.1%) -- guaranteed 3HKO
-1 204+ Atk Rhydon Drill Run vs. 248 HP / 0 Def Scrafty: 43-52 (25.1 - 30.4%) -- guaranteed 4HKO
Scrafty seriously needs 3 turns to kill it, assuming Rhydon doesn't bother to Protect. 1v1, sure, it wins, if Rhydon's partner does absolutely nothing for 3+ turns.
252+ SpA Aegislash-Blade Flash Cannon vs. 36 HP / 252 SpD Eviolite Rhydon: 138-164 (74.5 - 88.6%) -- guaranteed 2HKO
204+ Atk Rhydon Drill Run vs. 252 HP / 0 Def Aegislash-Shield: 102-122 (61 - 73%) -- guaranteed 2HKO
204+ Atk Rhydon Earthquake vs. 252 HP / 0 Def Aegislash-Shield: 96-114 (57.4 - 68.2%) -- guaranteed 2HKO
The calcs suggest you lose. However, a -Speed natured Rhydon is slower than Aegislash, so you'll be hitting it in Blade form (aside from TR, but you just outrun and 2HKO it anyway) for a solid OHKO.
252 Atk Tough Claws Mega Metagross Iron Head vs. 36 HP / 12 Def Eviolite Rhydon: 114-134 (61.6 - 72.4%) -- guaranteed 2HKO
204+ Atk Rhydon Drill Run vs. 4 HP / 0 Def Mega Metagross: 102-122 (65.3 - 78.2%) -- guaranteed 2HKO
They can both make a Sub and survive a hit.
252+ Atk Huge Power Mega Mawile Iron Head vs. 0 HP / 0 Def Eviolite Rhydon: 150-176 (83.3 - 97.7%) -- guaranteed 2HKO
Highest attack stat in the game + SE STAB = still not enough to kill a Bashful, untrained Rhydon.
252 SpA Thundurus Hidden Power Ice vs. 36 HP / 252 SpD Eviolite Rhydon: 56-68 (30.2 - 36.7%) -- 51.2% chance to 3HKO
204+ Atk Rhydon Rock Slide vs. 4 HP / 0 Def Thundurus: 134-162 (86.4 - 104.5%) -- 25% chance to OHKO
Bulky Thundy doesn't even kinda sorta budge the beast.
252+ Atk Landorus-T Earthquake vs. 36 HP / 12 Def Eviolite Rhydon: 84-102 (45.4 - 55.1%) -- 45.3% chance to 2HKO
-1 204+ Atk Rhydon Ice Punch vs. 4 HP / 0 Def Landorus-T: 136-160 (82.4 - 96.9%) -- guaranteed 2HKO
Does Landog really want to lose 82% min HP to MAYBE 2HKO Rhydon under ideal circumstances? If you switch in after the Intimidate, Landog can't do a damn thing.
252 Atk Gyarados Waterfall vs. 36 HP / 12 Def Eviolite Rhydon: 156-184 (84.3 - 99.4%) -- guaranteed 2HKO
4x SE STAB off of 125 base Attack, still not enough.
0 SpA Amoonguss Giga Drain vs. 36 HP / 252 SpD Eviolite Rhydon: 124-148 (67 - 80%) -- guaranteed 2HKO
204+ Atk Rhydon Ice Punch vs. 252 HP / 252+ Def Amoonguss: 82-98 (37.1 - 44.3%) -- guaranteed 3HKO
Yeah, even a STAB'd 4x SE hit isn't enough for Amoonguss to OHKO something with base 45 Sp. Def. Rhydon obviously loses in any situation.
252+ SpA Choice Specs Hydreigon Draco Meteor vs. 36 HP / 252 SpD Eviolite Rhydon: 147-174 (79.4 - 94%) -- guaranteed 2HKO
252 SpA Life Orb Protean Greninja Ice Beam vs. 36 HP / 252 SpD Eviolite Rhydon: 143-172 (77.2 - 92.9%) -- guaranteed 2HKO
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 36 HP / 252 SpD Eviolite Rhydon: 91-108 (49.1 - 58.3%) -- 96.9% chance to 2HKO
100 SpA Suicune Scald vs. 36 HP / 252 SpD Eviolite Rhydon: 156-184 (84.3 - 99.4%) -- guaranteed 2HKO
Even on the Special side, Rhydon is hard to OHKO without a Water/Grass move.
Water/Grass moves with STAB (or CHar-Y's Solarbeam) will obviously maim you assuming the user doesn't offensively suck like the above calcs.
C might be too high for it in my opinion maybe C- might be better. The only job it seems to really want to do is stop thundurus/zapdos from doing their thing against teams that are weak to them. Its HP and Defense stats are really good but its sp defense is terrible, which is not a good thing when you're weak to water, grass, and ice. If that wasn't bad enough, it also is weak a lot of common offensive typings such as; fighting, steel, ground, water, etc. and only really threatens steel types in return with its stab ground type moves. Also, your claim that it "takes less than 50% from anything that isn't 4x super effective against" is false.
252+ Atk Landorus-T Earthquake vs. 252 HP / 4 Def Eviolite Rhydon: 116-140 (54.7 - 66%) -- guaranteed 2HKO
252+ Atk Landorus-T Superpower vs. 252 HP / 4 Def Eviolite Rhydon: 94-112 (44.3 - 52.8%) -- 23.4% chance to 2HKO
252+ SpA Aegislash-Blade Flash Cannon vs. 252 HP / 252+ SpD Eviolite Rhydon: 128-152 (60.3 - 71.6%) -- guaranteed 2HKO
I only used moves that have only 2x damage against it but you get my point, its not as bulky as you claim it to be and it actually needs eviolite to survive the 2hko on superpower.
252+ Atk Landorus-T Superpower vs. 252 HP / 4 Def Rhydon: 138-164 (65 - 77.3%) -- guaranteed 2HKO
-1 252+ Atk Landorus-T Superpower vs. 252 HP / 4 Def Rhydon: 92-110 (43.3 - 51.8%) -- 10.9% chance to 2HKO
Disclaimer: I really don't know what you'd run on a rhydon so I went with a 252/4/252 spread going pure defense and went with a 252 aegislash, 0 EVs still does over 50% damage to that same build.
And while it does give M-Khan issues as well as a decent number of other megas that would give a charizard issues, landorus and aegislash are both common teammates to it.
Your EQ calcs are assuming Singles, not doubles; I'm not trying to be an ass, I made the same mistake like 4 times in this post alone. :P
252+ Atk Landorus-T Earthquake vs. 252 HP / 4 Def Eviolite Rhydon: 116-140 (54.7 - 66%) -- guaranteed 2HKO (singles)
252+ Atk Landorus-T Earthquake vs. 252 HP / 4 Def Eviolite Rhydon: 86-104 (40.5 - 49%) -- guaranteed 3HKO (Doubles)
tl;dr even in bad matchups Rhydon's a total dick to kill and it can tear a good chunk out of a lot of stuff.
But, no recovery + slow so it can be torn apart over 2-3 turns and it's very vulnerable to Intimidate and WoW. But it's still damn good support againt a lot of stuff; I just posted the stuff its disadvantaged against, nevermind the stuff like Mega Mence that's just fucked. C is fine.