Having Zero playable in the first Mega Man X game has been a dream for many after the famed Mega Man X 3: Zero Project by Justin3009 was made public at the end of January in 2012.  There have been strides to make this dream a reality.  The first of which was Megaman Z by Falchion22, and although Zero didn't have his trademark golden lush, static energy burst from dashing, and looking like a palette swap of X, the dialogue does change to Zero's POV and Wily capsules.  The second attempt was Mega Man X - Zero Playable by Programer Peru which took Zero's sprites from X3 and although it was a better looking Zero, it was canonically out of place and the dialogue was the same as the original game until skull_emperor_ came out with a text fix that modified all dialogue in the game borrowing heavily from Falchion22.  The third iteration came in the form of Mega Man X - Zero Playable Canonical Sprites with Programer Peru coming back again for a second run.  This is an upgrade from the first run: the dialogue has been revised and as the name suggests Zero's sprites are canonically correct.  A couple things that stand out are that Zero's main weapon still says X.Buster, the extra life is still X's head, and the Light capsules are still present but with dialogue that would fit Wily.  The fourth and most recent version is Mega Man X: Project Zero by CF.  This one takes the cake by far.  Explaining everything here about it would be too long so it is advised to read the description by clicking the link to that page.  So, this coupled with MSU-1 is pretty amazing.  Have fun!