I don't know the who, the where, the when, the how, or the whys of this, but since there are videos, I figured that it might be fun to share anyway. Some sprite hacking has been done on Mega Man Zero 3, enabling "other characters" to become playable. Those available are Model X, who, for all intents and purposes, makes a decent enough stand-in for X for my liking (but isn't letting him use the saber cheating a little?); Copy X in full armor, who has some unique movement styles like his slide-dash thing; a really screwed-up Vent; and a black Zero, which seems to be modeled a little more on the Mega Man X series style of Zero.
Again, I don't know where these originated from or how they're implemented, but one can assume ROM hacking may have been involved. These videos are simply here for your viewing pleasure, and conversation regarding where to acquire these ROMs will not be permitted. Feel free to discuss other related topics, such as characters you might like to play as in the Zero series or whatever, but take the discussion and distribution of ROMs elsewhere, lest we stop bringing stuff like this.
News credit: Protodude's Rockman Corner