The ninja master Koga has trained his Poison-types to cause you a shameful defeat. If you want to avoid that scenario, you may want to prepare your best Psychic and Ground-types.
Koga’s first Pokémon is Tentacruel, so put away your Ground-type for a moment. This sinister jellyfish can use Hydro Pump and Sludge Bomb for STAB, Mega Drain to steal HP from Ground-types, and Toxic to fatally poison your companions. Psychic and Electric-types are recommended.
For his second Pokémon, Koga may send out Venomoth. It can use Bug Buzz and Sludge Bomb for STAB, Psychic to hurt Fighting and Poison-types, and Mega Drain to overcome Rock and Ground-types. It’s easy prey for Psychic, Fire and Flying-types though.
Golbat you should be very familiar with. This one can use Fly to disappear for one turn, before inflicting STAB damage. It can also use U-Turn to deal damage and switch out, Leech Life to drain HP from Psychics, and Crunch for other purposes. Electric or Rock-types should do great here.
Muk is Koga’s trump card. It can use Sludge Bomb for STAB, Mega Drain to annoy Ground-types, Dark Pulse to punish Psychic-types, and Moonblast to counter Fighting and Dragon-types. Despite this, Ground and Psychic are recommended, unless you want to wall it with Steel-types, for instance.
Finally, there’s Weezing. It can use Sludge Bomb for STAB, Toxic to put a countdown on your Pokémon, Fire Blast to melt Steel-types, and Thunder to surprise Water and Flying-types. Ground and Psychic-types should have no problems here.