Koga is actually alright, but I feel Ariados is too weak a choice for him given its paltry bulk and inability to pull off the Spider Web + Baton Pass combo its meant to given its low speed. I made a few changes to him.
I think there is only one other trainer in the game with
Qwilfish, but I gave it to Koga here to take the place of Forretress and serve as a stronger lead. With Aqua Tail / Poison Jab / Explosion / Toxic Spikes, it gets the ball rolling with poisonous hazards. It can then attack or explode to make progress and deals with ground-types to an extent.
Gligar was once considered a fully-evolved pokemon and thus replaces Ariados. Its set is Earthquake / Swords Dance / Toxic / Baton Pass. This lets it spread status, covers steel types that might otherwise be immune, and allows it to stat pass attack to another member of the team. It being a poisonous scorpion fits thematically as the Gligar line has always been a bit of an honorary poison type flavor wise.
Muk is a staple. I left its moveset of Toxic / Screech / Minimize / Gunk shot unchanged as with black sludge, evasion, and toxic damage accumulating from the other two it can become problematic if you don't deal with it quickly.
Venomoth offers Koga a special attacker, but it has no offensive STAB attacks which is pitiful. I would change its ability to
Tinted Lens and give it a set of Signal Beam / Sludge Bomb / Sleep Powder / Psybeam. This gives Koga an alternative status affliction and gives his team some much-needed offensive firepower.
Crobat is a fitting ace, swooping in with high speed to pick up the pieces. I would amend its set to
Fly / Double Team /
Cross Poison / Quick Attack, which lets it snipe off your weakened team or offers a few additional turns for poison to take its toll between evasion and critical hits.