Those are just imposters. If you kill any Bowser up to 8-4 with fireballs, when they fall off the screen you'll see their true self, like a Goomba or Beetle, depending on the level. This could only be possible thanks to Bowser's "black magic" mentioned in the SMB manual...
Technically, the imposters shouldn't be considered Bowsers, since the real one is always in 8-4, but it's still safe to call them Bowser because they all have his powers and yield 5000 points when fireballed.
By the way, Super Mario 64 has three Bowser battles, but it's just the same Bowser coming back for more.