Nightmare

Nightmare is a creature infected by the X Parasites and is the eighth boss Samus faces aboard the Biologic Space Laboratories research station in Metroid Fusion.

Biography
Nightmare is "a bio-mechanical organism designed for military applications" and "possesses uncommon power and can manipulate gravity". When Samus leaves Sector 5 for the first time, she catches glimpses of the Nightmare flying around behind the large windows (possibly its containment tank), causing the entire room to shake violently. It also appears when she is entering and leaving Sector 5 while obtaining the Power Bombs, though it is never mentioned by Adam after any of these encounters. After Samus obtains the Plasma Beam, Adam detects an explosion in Sector 5 and tells Samus to investigate. When she arrives, Samus finds most of the sector in ruins. Exploring further, she eventually finds the X-infected Nightmare in a uniquely mechanized room. It is interesting to note that the Nightmare appears damaged. The damage adds to its nightmarish appearance, making it look decomposed and rotting.

Battle
This battle has 3 phases, not including its Core-X form.

Phase 1
During this phase, Nightmare will stay in front of Samus and shoot bursts of lasers from its arms at regular intervals. These bursts can be difficult for Samus to jump over as it shoots them in a spread. The Nightmare attempts to slowly follow Samus's movements.

Its weak point is the Gravity Manipulator - the glowing purple protrusion connected to its central body. Normally, its arms will be blocking it, so Samus must lure the Nightmare up by jumping high, then coming down and getting a shot or two at it using the charged Plasma Beam or an Ice Missile. Once it has been sufficiently damaged, the Nightmare enters its second phase.

Phase 2
The Nightmare's Gravity Manipulator starts running, lowering the height of Samus' jumps and the distance which her missiles may travel, forcing Samus to rely on her charged Plasma beam. The Nightmare will begin to move slowly back and forth; its eye sockets also begin to emit a slimy, green substance. After taking sufficient damage, the Gravity Manipulator is destroyed, returning gravity to normal. Nightmare's face mask then explodes, revealing its true face, upon which it enters its third and final phase.

Phase 3
The Nightmare will begin to move erratically around the room, becoming difficult to dodge. Its exposed face functions as its weak spot. Samus must utilize the ladder and the Space Jump to effectively avoid attacks and aim counterattacks. When the Nightmare (along with its Core-X) is defeated, Samus's Gravity Suit is restored.

Metroid: Other M
Nightmare appears in Metroid: Other M as a boss with two phases. One being airborne and the second having its organic face shown. After it is defeated, its corpse rests on the ground until the post-credits sequence; this may imply that the Federation moved it to the B.S.L. research station for its appearance in Metroid Fusion.

Trivia

 * It is unknown when exactly the Nightmare became an X-host, though it can be safely said that it was uninfected before Samus acquired the Ice Missiles, as it only began flying around erratically in Sector 5 after she obtained the concussive weapon (this would also explain the seemingly senseless destruction it caused in the sector afterwards).
 * The boss is found inside a room in which the background is composed of scraps of metal, machinery and cables. Whether or not the Nightmare caused this mess is unknown. Metroid Prime and Metroid Fusion: Prima's Official Strategy Guide calls the room an "electronic junkyard".
 * There is an enemy in the Nintendo DS game, Mario & Luigi: Partners in Time called a "Snoozorb." It shares similarities in body plan, face, and name (pertaining to sleeping). Snoozorbs attack by either shooting a laser beam or changing into a ball shape and rolling into the Bros.