Fake Bowsers are the first seven bosses of Super Mario Bros. They are common enemies that Bowser magically transformed into clones of him. They all attack the same way, except the first five don't throw hammers, as the last two and real Bowser do. They attack by breathing fire, and can also jump, in which it is possible to run under them. Sometimes there are bricks in the room, other times there may be Podoboos, and in the World 4 castle, a fire chain. All seven were defeated one by one in their castles, with the bridge chopped down sending them falling into the lava; alternately, Mario or Luigi can shoot them with five fireballs to defeat them. They will assume their true form as they die if defeated this way.
The World 1 Bowser is a Goomba. There is nothing special to his battle arena.
The World 2 Bowser is a Koopa. There are bricks that the big Mario can use to walk over him.
The World 3 Bowser is a Buzzy Beetle, and the only one of his species to be affected by fireballs. There are bricks hanging from the ceiling, which is more of an obstacle as the player can't jump on them.
The World 4 Bowser is a Spiny. There is a Podoboo and a fire stick on this bridge.
The World 5 Bowser is a Lakitu, and the arena is basically the same as the World 2, given that the castle is a tougher duplicate.
The World 6 Bowser is a Blooper, and throws hammers in addition to his usual attacks, making him difficult to jump over.
The World 7 Bowser is a Hammer Bro, and also throws hammers.
World 8 is where the real Bowser is fought, who also throws hammers.