The Fireball Cannon is an enemy that appears in A Link to the Past.
The first Fireball Cannons are found within the Swamp Palace early on in the dungeon. These Cannons are stationary objects that usually sit in the center of the room. They are initially completely harmless to Link as even if he walks right into them, he will not take any damage. Their only form of attack comes in shooting Fireballs out towards Link. However, their fireball attack is coordinated with Link's Master Sword. Whenever Link slashes his sword, the Cannon will immediately shoot out a Fireball. The Fireball will always come out of the side of the Cannon that is closest to Link.
The Fireballs shot out by Cannons will deal one full heart of damage to Link. Usually this isn't much of a threat, but often Fireball Cannons can be found alongside a variety of enemies, including Winder, Bari, and Stalfos. This combination of enemies makes it difficult for Link to get by without taking any damage.