Banzai Bills are giant-sized Bullet Bills that have shark-tooth smiles. They can cover nearly the entire screen! They first appeared in Super Mario World, where they were less common than their smaller cousins. Even though they were giant, a single stomp takes one out. They were immune to fireballs.
They also appeared in MKFusion and again common in many levels. A single stomp, again, defeats their massive shells. Sometimes they come in groups of two or three, as seen in the Mystic Forest, where your only choice is to duck! Other times they're paired with Klampons.
Trivia: This is the only Bullet Bill variety that isn't fired from cannons.