Most custom bosses come in the form of this
As for the graphics, I gave Kamek's Island a quick play and the Birdos' graphics (specifically in "Birdo Bros Castle") look the same as the ones included in the "tilemaps" folder of SpriteTool.
Read post more clearly, and it looks like the Birdos included in SpriteTool are just "common enemies", which is kinda stupid. I never would have guessed anyone would even THINK to use Birdos like that. Try PM'ing the creator
the people who watch hentai as an anime, and not just as a porn, are the superior race