Rare mounts spawn time

Hey I've heard that Poseidus, Aeonaxx and the rest of the rare squad spawns in certain time and it respawns in the matter of hours. So how does that work? How do I know when someone killed them so I can calculate the next respawn time? It's really frustrating for me, so if anyone can help it would be rad. Thanks.

Most of them have different respawn timers. You can't know when someone killed them unless that someone reports the kill. It's frustrating to everyone

Aeonaxx has 8 hours respawn time. I've killed Poseidus aswell, then camped him for 4 days - no luck... so i can't tell a time for it.