Rhydon issues

Rock Smash is a bad quick move for Rhydon (slow and no stab). I evolved one with RS/SE 95.6% IVs and not powered, i still working for a MS/EQ or SE then i will max him out.

It will do fine against anything weak to fight.

Rock Smash vs Tyranitar = 2x Super effective = 1.56x damage
Mud Slap vs Tyranitar = STAB and Super effective = 1.25 x 1.25 = 1.56x damage

Also Mud Slap has a slightly shorter cooldown, which shouldn't matter in almost all cases since you'll be dodging, but it's still an advantage.

Rock Smash is only better when it is SE and Mud Slap is resisted, because then Rock Smash would do 1.25x damage while Mud Slap would do 1x damage (1.25x STAB x 0.8 resistance = 1)

Same goes for Snorlax.
Rock Smash being super effective results in 1.25x damage, and Mud Slap having STAB results in 1.25x damage, except that you'd lose SE against Pokemon that aren't SE to fighting whereas you'd keep the STAB damage, and you also have the higher potential for damage due to being able to stack more bonuses (STAB and potentially 2SE).

So Rock Smash isn't the optimal moveset for "countering" Snorlax or Tyranitar, it's still only on par with Mud Slap, and slightly worse in a vacuum i.e. assuming you only spam moves and don't dodge, the 0.06 lower cooldown of Mud Slap will result in slightly higher DPS.

When Gen 2 comes out, Ampharos is predicted to have high stats, probably making it the best Electric type, and then you'd want a double Ground move Rhydon for it over one with Rock Smash.

Megahorn is not a bad charge move at all!

You can use it as a gym defender. Most people will try to attack it with Grass types, which is exactly why Mehahorn will perfectly sabotage their plans!

Rhyhorn and even Ryhdons are very common here. I have caught or evolved 12 Rhydons and only 1 had Mud Slap/Earthquake. Only 1 had Mud Slap/Stone Edge. Very unfortunate. I still have 350 candies and a few more Rhyhorns around 82IV but waiting for a better one.

6 of them have RockSmash
4 of my Mudslap guys have Megahorn (sucks)
Only 2 of all my guys have Earthquake.

I have one 96% that I slowly keep powering up that has Rock Smash and Stone Edge. He's my Snorlax killer and he does quite well. I don't really use him on anyone else but it's nice having a good Ryhdon that can easily take down a Snorlex.

Just gotta keep at it and hope for a better one soon...

The only Rhydon I ever evolved is now 2676 CP with Rock Smash + Earthquake (I powered him a little bit). He's still an absolute tank and he can wipe out two high CP Snorlaxes in a row - yeah I can only put one Rock Smash in between the opponents' quick moves, but it's enough. Plus Mud Slap isn't even that much faster. Rhydon will always be a beastly mon regardless of its quick move.

If you have 8 Rhydons you must have plenty of Rhyhorn candy supply. So I'd suggest you can always power up the RS+EQ one for now, have fun with it and keep trying to evolve a perfect one, or just wait if you don't have too much stardust to spend.

Rhydon with Megahorn does surprisingly well attacking Exeggutors (1.56x damage). One caveat though: avoid Solar Beam defenders if possible. Mud Slap is resisted by Exeggutor but it gets STAB so it balances out.