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Metroid - Did You Know Gaming? Feat. WeeklyTubeShow
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Sega's Toshihiro Nagoshi revealed in an interview, where he talked about F-Zero GX, that Sega had also pitched a Metroid game to Nintendo. However, Nintendo had obviously turned down Sega's pitch.
Contributed by PirateGoofy
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.
Contributed by barryc100588
There were plans to make a Metriod game for the Nintendo 64, however the series co-creator, Yoshio Sakamoto, said that he was unable to find a way to control Samus, and when he gave an unknown company the right to make the game, they turned it down thinking it wouldn't live up to Super Metroid.
Contributed by gamemaster1991
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: Dark Moon.
Contributed by Boyobmas
Samus' Morph Ball exists only because Nintendo's programmers couldn't animate Samus getting on all fours and crawling through tight spots.
Contributed by Stryker94
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!"
Contributed by Pogue-Mahone
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'."
Contributed by Pogue-Mahone
In F-Zero X for the N64, the pilot 'Jody Summer' has an impeccable resemblance to the ending-screen version of Samus from the original Metroid.
Contributed by cross307