A Bang is an unused creature in Super Metroid. It is referred to as a "Possessor-type" enemy. Bang will float around a room until attacked. When attacked, it will flash and grow larger. After a certain number of attacks, Bang will explode, killing itself - but at the same time, unleashing all of the damage that Samus has inflicted on it in a massive shockwave. At later areas of 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, and usually a simple Morph Ball evasion is enough to avoid getting hit.
It can also "possess" other enemies, but what exactly this entails is unclear. 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.