From Bulbapedia, the community-driven Pokémon encyclopedia.
This is a list of Pokémon by their date of revelation. Whether or not it is mainspace material I don't know; I want to keep it, even if it's not.
The first column is when a silhouette is given, it is seen without being named or accidentally revealed, or it is seen in a battle screenshot with no other elaboration. The second column is the official release, which includes leaks of official sources, Pokémon Sunday/Pokémon Smash!, CoroCoro, etc. There are two columns because often multiple Pokémon are revealed at once, so this column can further break it down.
Generation III and IV are made by Han Ji-Wan and someone needs to go through the Bulbanews archives and find articles to link to. They're missing quite a few Pokémon, most notably event legendary Pokémon that are only revealed post-release. Currently it is just based on Serebii's news archives.
I highly doubt that I would go back to Generation II or before, as these releases are less clear. It might be nice if we were to include a "release" trivia piece on the Pokémon pages that are on this list.
I'm considering redoing the style of table used so that it uses their minisprites rather than their full sprites. I could list the Pokémon which I compressed under "everything else", but they are all the same, and their date is the release date of the games, so I don't plan to unless someone requests it (and convinces me to do it).