James traded Victreebel for a Weepinbell from the Magikarp Salesman during one of his scams. That Weepinbell evolved into Victreebel soon after, they both were kicked away by their owners and bumped into each other. As a result, they fell in love and ran away together.
James bought Gyarados when it was still a Magikarp. When James threw Magikarp into the ocean, it evolved into Gyarados. Eventually, it used Dragon Rage hurling Ash, Misty, Brock, and Team Rocket away, and it was never seen again.