Let's get some hilariously gimmicky and niche counters to specific Pokémon.
I'll start:
Tortuga (Torterra) (M) @ Expert Belt
Ability: Shell Armor
Shiny: Yes
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Wood Hammer
- Earthquake
- Synthesis
- Rock Slide

This counters Rotom-W great. It outspeeds non-choicescarf Rotom-W, and can deal heavy damage while preventing a volt switch out.