E.M.M.I.

The Evil Main Monster Idiot, of E.M.M.I., is a very powerful robot that likes to harass samus wherever she goes on this planet for no reason other than to be spooky and make the game more tense.

Background
The E.M.M.I were created for "Research Applications" by the Galatactic Federation, who, in all media prior, was depicted as being hopelessly incompetent, needing to constantly beg Samus for help whenever they were getting bullied by the Space Pirates, who now all of a sudden are a big threat in the universe. The E.M.M.Is' armor plating are created with nearly indestructible materials, that can resist pretty much any conventional weaponry. Where was this incredibly useful material with endless Military Applications when they were in an all-out war? I dunno, maybe they just found it really later on in the series, there surely were no particular threats, forces, or materials that needed this level of gear to deal with at the time beforehand.