The King of the Koopas and the final boss of Super Mario Bros. Bowser waits at the end of World 8-4, breathing fire and leaping across the bridge above a lava pit. Unlike the fake Bowsers in previous castles, this is the real deal — he requires 5 fireballs to defeat, or Mario must reach the axe behind him to drop him into the lava.
Hit him with 5 fireballs as Fire Mario to defeat him in place, or run past him and reach the axe at the end of the bridge to cut the bridge and drop him into the lava. Running under him when he jumps is the key to reaching the axe without fire power.