This page contains a short overview of all world bosses in game, including those that were nerfed, removed, or just not updated for the level cap. These bosses are considered raid bosses since it takes more than a party of 5 players to down them. Blizzard seems to have lost interest for a time in the idea of world bosses as none had been added between the Burning Crusade expansion and Mists of Pandaria.
At the time of the testing, Azuregos/ Lord Kazzak used to spawn a little earlier than the world dragons. The dragons spawn one unique per the 4 spawn locations, typically all at the same time. One continent being down without the other desyncs the dragons, possibly making them spawn duplicates. (Thus occasionally spawning 2 of exactly the same bosses in different locations).
After the Burning Crusade came out, two new world raid bosses were introduced, Doom Lord Kazzak and Doomwalker (there were others initially shown in databases as ?? bosses, including a rare spawn from a quest in Blade's Edge in which you can spawn a raid boss who flies away, but is unaggro-able. But currently it is accepted that there are 2 BC world raid bosses). The popularity of the two was short-lived. After a couple months it became apparent that loot off of Karazhan was on par or better, while not having the additional faction vs. faction griefing factor.
In Patch 2.2, world bosses were often left untouched for long periods of time. There were occasional guilds that use them as either practice for raiding, or under-geared guilds trying to gear up. But with a couple of announcements by blue post that they did not plan to add additional world raid bosses, the days of a new world raid boss came to an end.
In some rare elites were added that were more like outdoor dungeon bosses that usually required a good 5-man group or more to kill and loot. They technically were not raid bosses, but the closest since Burning Crusade to a world boss. 2b1af7f3a8