Koga is the 5th Gym Leader in the original pokemon series. His speciality is poison-type pokemon which makes him somewhat challenging. About the time you reach him, Gym Leaders are no longer cake-walks.
I also remember his anime appearance with his poisonous bug pokemon as well. Not a bad episode.
In the 2nd generation of games, he gets promoted to the Pokemon League. His daughter takes over the gym when he's gone. I loved the 2nd part of the game where you catch up with all the people from the first game.