A large black bullet fired from Bill Blasters — cannons embedded in the terrain. Bullet Bills travel in a perfectly straight horizontal line at a fixed speed and will keep going until they hit something or scroll off screen. They appear in large numbers in later worlds.
Jump on top of one to defeat it for 200 points, or hit it with a fireball. The easiest approach is to simply duck or jump over them — they travel in a straight line so they are very predictable.