Metroid 64 is the title given to a possibly existing beta for either Metroid Prime that started on the N64 or an entirely different unreleased game.
Ever since the concept of a Metroid game on the N64, several theories have come up on what the game would have been. here is a small list of the most common and believed theories.
Metroid Prime beta
Several people believe that it was simply a beta for Metroid Prime which came out on the GameCube years later. No real evidence of this has really come up but there is not much against it so it is a (minor) possibility.
New game by Nintendo
A big myth is that Nintendo where working on an original game which would have been a third person game. However there is lots of evidence that this never happened (see inaccuracies).
New game by a third party company
Another popular theory is that Nintendo approached an unnamed third party to make the game. This one has an element of truth but disproves the theory.
Also some people say that the game was in development for 64DD but since the system died so quickly it was cancelled. No evidence of this myth is found online.
A video by Yuriofwind talks about how Nintendo where trying to make a Metroid game on the N64 but simply had no ideas therefor the game was put out of development for other projects
In an interview found by wikitroid Yoshio Sakamoto talks about the game. This interview confirms that a third party was contacted by rejected it because they had no idea what to do and that Nintendo tried as well but where unfruitful.
To sum up, it is highly unlikely that the game was even made but fans still hold hope that a beta could be released someday. Also to note I only made this page because it was on the article requests page forever so...