Nightmare is a military bioweapon created by the Galactic Federation to manipulate gravity with ease. It does this with the manipulator upon its 'tail'. It appears in Metroid: Other M and Metroid Fusion, in the latter being infected by an X-parasite.
Nightmare as the unique ability to control gravity, manipulating it using its 'tail'. It also has many inbuilt weapons to combat foes, further evidence of it being for military purposes. Samus' shots are also pulled to its tail like a tractor beam in Other M.
Nightmare has appeared twice in the series so far.
 Metroid: Other M
 Metroid Fusion
Related ThreadsNightmare or Dreameater - last post by @ Jun 6, 2009
the move nightmare - last post by @ Feb 29, 2008
nightmare question - last post by @ Nov 4, 2007
Nightmare - last post by @ Jul 5, 2007
SOS Please! The Nightmare 17F - last post by @ Nov 26, 2010