From the Super Mario Wiki
The Air Cannon is an enemy that can only be found inside the Airway in Mario & Luigi: Bowser's Inside Story when pursuing the Dark Star. It is a teal-colored egg creature that has been influenced by the Dark Star, and operates a red Bill Blaster-like cannon.
From the cannon, the egg creature will shoot white, airy Bullet Bills in two different ways. In the first way, the Air Cannon will shoot some Bullet Bills randomly at Mario or Luigi. The Bullet Bills may come straight on or fly in a small loop. Occasionally, they may "disappear," but they actually turn invisible. With key timing, the plumbers will need to jump over the Bullet Bills to avoid taking damage. Their second attack involves shooting three Bullet Bills into the air. The Bullet Bills perform a loop-de-loop and head towards one of the bros. However, as the Bullet Bills come down, they will turn invisible. The bro will need to hammer the Bullet Bills with key timing in order to avoid taking damage. Once the cannon is defeated, the creature operating it will flee, but it will come back in a few turns if the battle has not ended, wielding a new cannon.
Names in other languages
||Cañón de Aire
||From aerobio (aerobic).
||From aéro- (in the air) and Bill Boum, French name for Bill Blaster.
||"Aero" (Air),"Cannone" (Cannon)