The Varia Suit upgrade is actually a mistranslation from Japanese. It was supposed to be "Barrier" Suit, but since Barrier is written roughly as Baria (バリア) in Japanese, the translators mistakenly wrote it as Varia instead, and the name stuck.
According to some concept art posted online, Next Level Games were working on a prototype game for the Metroid series but abandoned the idea when they were put to work on Luigi's Mansion 2.
Samus' Morph Ball exists only because Nintendo's programmers couldn't animate Samus getting on all fours and crawling through tight spots.
According to Hiroji Kiyotake, the Metroid's home planet, SR388, was named after a popular Yamaha SR 400 Series Motorcycle engine.
"...while it was called 400cc, you didn't have any choice but 388cc, but it was written like that for the time being, and that's how it came out!"
In an interview, Metroid character designer Hiroji Kiyotake said that the name "Metroid" came from combining the words "android" and "metro subway."
"There was one more staff member, and the two of us decided on it. We attached 'android' to the "metro subway" and that's how we got 'Metroid'."