All of Koga’s Poison-types carry Protect, while can only mean one thing. Toxic stalling. Indeed, besides Venomoth, all his Pokémon can use Toxic. If you want to avoid Toxic, Steel-types are ideal. If you have Poison-types with a beneficial second Type, like Ground, they can be useful as well.
Weezing can use Sludge Bomb and Explosion in addition to Protect and Toxic. Explosion is a stronger version of Self-Destruct, dealing lots of damage, but KOing Weezing in the process. Ground or Psychic-types are great offensive choices, while Steel is good for defense.
Next can be Venomoth, which can use STAB-boosted Bug Buzz and Sludge Bomb, plus Psychic to hurt other Poison-types trying to wall it. Fire, Flying, Psychic and Rock-types are great here. Steel and Poison are less useful since Venomoth can’t inflict Toxic.
Koga also has a Golbat. In addition to the two signature moves, it can use Fly to disappear for a turn, before dealing damage on the next. Plus Leech Life, which is a strong Bug-type move that steals HP. Electric, Rock and Ice are recommended for attacking with, Steel and Poison to negate Toxic.
Finally, Koga’s Muk can use a STAB-boosted Sludge Bomb, plus Moonblast to, er, be a hipster or something? We’re honestly out of ideas. Again, go with Ground or Psychic for offense, or Steel or Poison for defense. If you haven’t figured it out, the fully evolved Nidoran are really good choices.
Neutralise Koga’s poison and he’ll give you his Soul Badge, plus his family heirloom, TM27 Toxic. If you haven’t experienced it before, Toxic inflicts a special type of Poison that gets worse and worse as time goes on. In fact, if untreated, a Pokémon at full HP will be reduced to 1/16 HP when Turn 5 begins and will faint on Turn 6.