Koga is undoubtedly the most handsomest gym leader ever...*sighs*. Anywaaaaaay, he uses Poison Pokémon like Grimer, Muk and all that. *cracks knuckles* Although he's all handsome and that, his gym(in the game) is a very annoying place with those invisible walls and all. *grumbles* It took me quite a while to get to him, but if you adjust the contrast a little, you can see the invisible walls a little bit. -_-; But it's still annoying, nonetheless.

Fortunately, he's a snap! Just use any psychic or ground Pokémon and it'll be okay before you know it. However bring a couple of antidotes, in case your Pokémon gets poisoned.

The other trainers in his gym(mostly Jugglers & Pokémaniacs) uses Psychic Pokémon(ala Hypno...and Kadabra @_@), so you might want to bring a Parasect along in case.

Yellow players have it easier since it follows the show and in the show Koga has a Venonat and a Venomoth. So eventually in Yellow, he has THREE Venonats and a Venomoth. Since they're part bug, they can be taken down by any fire Pokémon(like Charmeleon and Flareon - if you evolved your Eevee into Flareon, that is).

If you win, you get the Soul Badge which raises your Pokémon's defense and allows you to use HM03, Surf, anytime.