Allow me to introduce the newest addings to Warcraft Mounts: Recent Additions and Upcoming Additions. Recent Additions showcases mounts that were added to World of Warcraft in the last major patch; Upcoming Additions highlights what we know about mounts coming in the next major patch.
In addition, after a lot of research (and, frankly, a lot of guessing), I’ve added a field to each mount indicating the patch in which it was introduced — as least, as far as I can tell. I stuck with major patches rather that dates, so for seasonal mounts and the code & card mounts I just used with the patch before the event/booster release. I did have to piece together a lot of random bits to try to figure out some of the mounts, so if you see any mistakes please let me know.
Finally, the Seasonal Mounts now have their own page since, with the addition of Reindeer from the Feast of Winter Veil, they were crowding out the other mounts. Reindeer, of course. are not true mounts — but as an effect you can use to transform your normal mount they sure are cool!