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According to the MtG wiki, the symbol 
is a "Generic mana symbol", specifically representing
Generic mana in costs that only can be paid with mana from a source that can produce two or more colors of mana.
However, I can't find any reference to it in the Standard Rules, and I've never seen or heard of it before.
Where does this symbol come from? What cards does it appear on? And what does it mean, exactly?
There is exactly one card with the {Z} symbol: The Ultimate Nightmare of Wizards of the Coast® Customer Service. It is a silver bordered card from Unglued, and is not legal in regular play. On this card, Z functions exactly the same way X normally does, and the different letter merely serves to distinguish it from the X and Y already present on the card.
The wiki description comes from Experiment Five a "playtest card" from the recent Mystery Boosters. These are not real cards and were not intended for actual play (even more so than the silver bordered cards, which were intended for play, just not regular play)
As to what it means, it means exactly what it says. The playtest cards have no actual rules support. All interactions have to be interpreted by players from the cards themselves.
