Is it Boss Bass? Is it Big Bertha? Is it a giant Cheep Cheep?
It’s all three!
Yeah, Nintendo created three new giant fish characters using IDENTICAL sprites in Super Mario Bros. 3, and the only way to tell them apart is from context: Boss Bass chases Mario along the surface of the water and tries to eat the heroic plumber. Big Bertha is seen exclusively UNDERwater, constantly spitting out and re-engulfing the Mini Cheep Cheep. And any giant fish that shows up in World 4, Giant Land, is probably just supposed to be a giant Cheep Cheep, to go along with the Giant Koopa Troopas and Giant Little Goombas.
It’s a hell of a way to save on resources, just using the exact same graphic for 3 separate characters.
I don’t think either Big Bertha or Boss Bass have appeared outside of SMB3, unless you count the large woman in the red spiky dress in the live-action Super Mario Bros. movie who is named Big Bertha, but is in no way a giant red fish.
God, that movie is weird.