- Northrend can have four (could be three, but i've seen enough to think it's four) rares up at any time. Rares do not have a "one per zone" requirement, nor do they share spawns (other than vyragosa and tlpd).
- Northrend rares can respawn within a range of 6-24 hours. Your server will roll a number 6-24 from the moment a rare is killed, and assigns that number as the timer.
- Northrend rares with multiple spawns do not have a "favorite" spawn point. Spawn location is entirely random.
- If your server has four rares alive, the timers for all other rares continue ticking down until one of the existing rares is killed or tamed. At this point, any timer that has elapsed can fire and spawn its mob.
What this means is that respawn times are heavily-dependent on server population and time of day. The more rares that are being killed, the more that are free to spawn. I'm obviously not 100% on any of this, but from everything I've seen, I'd bet the farm that this is how Northrend rares function
