CTNC
Doesn't know how to attack
I thought Onyx too, but Rhydon is bulkier and can use Whirlpool for a Toxic + Whirlpool Combo that may work better than Perish Trap...Although this maybe an odd choice given its lack of immunities and resistances that its evolution has but ALL Rock/Grounds have this issue, Onix. It becomes bulkier than Steelix and is faster too. In a Sandstorm it maybe viable.
Onix @ Pre-Evo Stone
Impish
Sturdy
252 Def/126 SpD/126 Atk/4 HP
-Stealth Rock
-Rock Slide/Stone Edge
-Toxic
-Earthquake/Protect
Point is to set up SR at least and status and Stone Edge and Earthquake is for STAB so you are not a sitting duck. Switch EQ with Protect if you want to try to get one last turn with Toxic. Again out of the blue but creative.
Edit:
I heard that works but if it doesn't work Toxic Trap is out of the question...Afaik, Whirlpool shouldn´t be possible on Rhydon, as you can´t pokeshift Pokemon that know HM moves, right? Or does it work if done like this?: HGSS --> DPPt --> BW
Also, I see alot of BL NFEs and a bulky UU tier