Bombaman is a Dark Humanoid guard and one of the Dark Summoner's guards. It does not transform into any other guard.
Bombaman is first encountered in Dr.X's lab. "Exploring the ground floor" will bring up two door guards, like with Mickia in the Sky tower. These door guards are Bombamen. They both appear at Rank 45, with different stats. The first Bombaman is simple to defeat; the second one is harder. That Bombaman is faster and stronger. They are useful in the game, defeating some tough ground and grass guards like Traruz, Mycena and others with strong stats.