This is a bit unusual. I can't recall in recent memory any Pokemon for a new Generation being revealed before the current one was complete. I'm certain Magiana will not be in Z so that can only mean it's the first Gen VII Pokemon to be revealed. Either way, I'm intrigued.
Ho-Oh appeared in the very first episode of the anime, broadcasting April, 1997. Pokemon Red and Blue didn't see worldwide release until September, 1998. Yellow was released in Japan that same month, with the rest of the world getting it starting October, 1999.
Kecleon first appeared in the anime as well,
The Kecleon Caper, which broadcast July 2001, the month Crystal was released in the US. You could argue that since Gen II was already technically finished in Japan, Kecleon doesn't count.
Munchlax, the first Gen IV Pokemon, was revealed to the public May, 2004. FireRed and LeafGreen were released the same year, Japan in January, and worldwide starting September. September 2004 was also when Emerald was released in Japan, worldwide starting May, 2005.
Zorua and Zoroark were revealed February, 2010, in the middle of HeartGold and SoulSilver's release. Japan had gotten the games September of 2009 (and Korea got them the same month as the Gen V reveal), but the rest of the world didn't get the games until March. Again, you could argue that Gen IV was finished in Japan prior to the unveiling.
Gen VI is actually the odd one out here, likely due to Gen V's unique release schedule and game listing. The first Gen VI Pokemon were revealed January, 2013, two months after the last release of Black 2 and White 2, in Korea, November, 2012.
There is a precedent for unveiling the new Generation either before or during the close of the old one. Arguably, only Kalos breaks that trend. Moreover, Nintendo also has a tendency to release new movies featuring new Pokemon in the build up to next Gen release:
M02 (
2000) was released July 1999, Gold and Silver were released November, 1999.
M05 (
Latios & Latias) was released July, 2002, Ruby and Sapphire were released November, 2002.
M07 (
Destiny Deoxys), M08 (
Lucario and the Mystery of Mew), and M09 (
Ranger and the Temple of the Sea) were released July in 2004, 2005, and 2006, respectively. Diamond and Pearl were released September, 2006.
M13 (
Zoroark: Master of Illusions) was released July, 2010, Black and White were released September, 2010.
Kalos, again, breaks the trend. M16 (
Genesect and the Legend Awakened) did not feature any Gen VI Pokemon. However, it did feature Mega Mewtwo Y, and was released July 2013, while X and Y were released October, 2013.
M19 will see release in July of this year. I am of the opinion that we'll see M20 next year, and that the Seventh Generation will release that Autumn.
... Apologies for how long and obsessive that turned out to be. I didn't think it'd be half this length in my head.