marilli
With you
If you're going to elect a bulky Psychic-type with SR as its primary job, you can never ever forget about Uxie. If has SR, and a combination of U-turn, Dual Screens, Memento, and Heal Bell ensures that Uxie's going to help out a set-up sweeper for sure. Uxie can take most neutral STAB and laugh at it, while Dark-types like Honchkrow tend to have trouble with Rhydon. (Of course, unless the Honchkrow is mixed with HP Grass, in which case uxie usually has it easier because they tend to run sucker punch / hp grass / superpower / heat wave or some variant of it)
Outside of Uxie and few others, It's really hard to think of a good SR user off the top of my head that has synergy with Rhydon. SR users are now mostly limited to Ground, Rock, and Steel-type Pokemon, and additional Rock or Ground Pokemon is not something we want to add at this early point of teambuilding. Steel-types in RU are either Rock / Ground-typed anyways on top (Steelix, Aggron, etc.), or doesn't get SR (Klingklang, Magneton). Only exception on top of my head is Eviolite Ferroseed, which is capable of providing both SR and Spikes; however, it should stick to only one as that makes Leech Seed incompatible, and recovery-less eviolite mons tend to suck a lot. Ferroseed has decent synergy with Rhydon as well (Ferro resists Grass and Water, and Rhydon resists Fire)
If we ever need to go with a Spinner, I suggest either Hitmons in this slot. Spike-stacking offense should be, by definition, offensive, and defensive spinners like Claydol (big momentum loser), and somewhat Cryogonal (yeah his ice beams actually somewhat hurt, I guess, but he needs to be Specially Defensive to actually work) is IMO defeating the purpose of the team. These are the best offensive spinners that don't have either a Ground or Rock typing. (e.i. I eliminated Armaldo, Sandshrew, and Kabutops off the bat. Kabutops is pretty cool spinner, though.) Hitmonlee hits harder, but Hitmonchan can also elect to run Ice Punch in order to lure out Gligar and hit it hard, scare it off next turn, etc.
Also, we need solid counters for both Claydol and Gligar as they're the No. 1 counter for offensive Rhydon.
Outside of Uxie and few others, It's really hard to think of a good SR user off the top of my head that has synergy with Rhydon. SR users are now mostly limited to Ground, Rock, and Steel-type Pokemon, and additional Rock or Ground Pokemon is not something we want to add at this early point of teambuilding. Steel-types in RU are either Rock / Ground-typed anyways on top (Steelix, Aggron, etc.), or doesn't get SR (Klingklang, Magneton). Only exception on top of my head is Eviolite Ferroseed, which is capable of providing both SR and Spikes; however, it should stick to only one as that makes Leech Seed incompatible, and recovery-less eviolite mons tend to suck a lot. Ferroseed has decent synergy with Rhydon as well (Ferro resists Grass and Water, and Rhydon resists Fire)
If we ever need to go with a Spinner, I suggest either Hitmons in this slot. Spike-stacking offense should be, by definition, offensive, and defensive spinners like Claydol (big momentum loser), and somewhat Cryogonal (yeah his ice beams actually somewhat hurt, I guess, but he needs to be Specially Defensive to actually work) is IMO defeating the purpose of the team. These are the best offensive spinners that don't have either a Ground or Rock typing. (e.i. I eliminated Armaldo, Sandshrew, and Kabutops off the bat. Kabutops is pretty cool spinner, though.) Hitmonlee hits harder, but Hitmonchan can also elect to run Ice Punch in order to lure out Gligar and hit it hard, scare it off next turn, etc.
Also, we need solid counters for both Claydol and Gligar as they're the No. 1 counter for offensive Rhydon.