When X/Y came out, Hoopa, Diancie, and Volcanion weren't known about for a significant amount of time following the game's release, until they were datamined, and following that, I believe there was speculation as to whether they were Pokemon that were to be released in an upcoming event, or if they were scrapped Pokemon left in the code.
Has this happened with any other Pokemon, whether through official means or otherwise? For example, Arceus wasn't officially released until 2009, while D/P came out in 2006. In what capacity was Arceus known about? Was it datamined or leaked? Revealed through promotional material or the anime?
I'm especially curious about pre-Gen 6.
I'd like to specifically exclude mid-generation introductions like Black/White Kyruem, Poiple/Naganadel, etc. I believe they were revealed before they came out, but after the release of the first game in their generation, as they are not obtainable in the initial releases.
