The World of Warcraft addon WorldBossTimers which is used to track and share respawn timers for world bosses and other rare spawns that drop mounts.
Curse addon site: https://www.curseforge.com/wow/addons/worldbosstimers
Curse project site: https://wow.curseforge.com/projects/worldbosstimers
If something is broken or you have a feature request, feel free to add an issue or comment on the Curse addon site.