Beta Drone

From Wikitroid, the Metroid wiki

The Beta Drone (also called Shadow Drone) is an invisible boss that Samus cannot lock onto, unless she uses the x-ray visor, which you normally can't get at that point without sequence breaking. It is encountered before you recieve the Power Bombs in the Phazon Mines. The most effective weapon is a charged Wave Beam shot, though if Missiles are plentiful, a cheap strategy is simply to use the Wavebuster until the mechanoid has been destroyed. It can be quite a tough fight and it is one of the only bosses which cannot be scanned. After you beat it you must go through an electrical maze to gain the Power Bombs.