A Bang is an unused creature in Super Metroid. It is referred to as a "Possessor-type" enemy, as its pieces are made up of separate entities. The name comes from the game's internal code.
A Bang will float around a room, slowly following Samus, until it is attacked. Samus will take very minor damage when coming into contact with a Bang, but this is not the main danger of this enemy. When attacked, a Bang will flash and grow larger. After 5 attacks, the Bang will explode, killing itself (hence the name) - but at the same time, unleashing all of the damage that Samus has inflicted on it in a massive shockwave. This shockwave takes the form of a charged version of the beam Samus is equipped with, but the damage varies based on what weapon she used to kill it. Later in the game where the beam's power is quite deadly, getting hit by the backlash from a Bang can be near-fatal. Luckily, Bang's explosive burst does not track Samus after being released, so a simple Morph Ball evasion is usually enough to avoid getting hit. The released shockwave can hit other enemies in addition to Samus, including other Bangs, dangerously compounding the damage the second Bang will release upon death.
A Bang will be killed after being hit 5 times, regardless of the amount of damage dealt to it. If they are hit while growing, this will not count towards the hits needed to kill it, but the damage will still be added to the Bang's death shockwave. Any projectile weapon will work, including Bombs, but other weapons, such as the Screw Attack or Power Bombs, will not work. Bangs cannot be frozen, but they will absorb the extra damage the Ice Beam deals.
It bears a resemblance to Metroid Prime's nucleus from the end of the first Prime game. It also resembles the Phaazoid species seen in Metroid Prime 3: Corruption, and the Yameba in Metroid Fusion, the latter possibly being an infected and/or X-mimicked version of the Bang.