kappas aren't generally perceived as big and strong and angry however...their usually puny and green troublemakers. If you wanted to picture an imp...think of when your characters would be turned into imps on final fantasy 6 (3).
i believe that nintendo more based golduck from pokemon to be like the kappa than they did with bowser...that is to say if they even tried to base bowser off a kappa which i can't understand how or why. The only similarities between a kappa and bowser are that they have big green shells....except bowser's is spiky. Your picture of an angry kappa is sometimes rare as they are not normally big, strong, and scary in mythology. (WOW I LIKE HOW OFF TOPIC WE ARE COMPARING KOOPAS TO DUCKS OR TURTLES TO BOWSER AND KAPPAS) also, going back to pokemon, golduck's japanese name is indeed "Kappa"....bet not many of you guys knew that...lol