The Originals

Are they on a rotation? Ysondre and Emeriss don't seem to be spawning.

TomTom-Waypoints for all client languages:

/way #76 49.21 82.97 Azuregos (Azshara)
/way #47 44.82 39.65 Emeriss (Duskwood)
/way #26 63.56 27.18 Lethon (Hinterlands)
/way #17 32.26 49.61 Lord Kazzak (Blasted Lands)
/way #63 93.82 40.56 Taerar (Ashenvale)
/way #69 50.63 12.11 Ysondre (Feralas)
/cway

They are not all available at the same time. Look at your global world map, there is an icon like it is for rares when it is up.

Not sure about a rotation, today's bosses are Azuregos, Lethon and Lord Kazzak.

[npc=121820] in Azshara and [npc=121818] in Blasted Lands are ALWAYS up.

The rest of the bosses are called Dragons of Nightmare and they rotate. 4 different dragons, 4 different zones, and only one of them can be up. They change DAILY.

Rotation: [npc=121912] (Feralas) - [npc=121821] (Hinterlands) - [npc=121913] (Duskwood) - [npc=121911] (Ashenvale)
And so on.

If you are unsure what Dragon is up, simply just open your map and look for the world boss icon. This screenshot is with basic UI, no addons, so the icons are available for everyone: https://i.imgur.com/nR5GrPV.png . If you hover over the icon, it will even tell you the name of the boss and how long it will be up.