While we're doing famous Everquest rumors, a dev (Prathun) debunked any number of theories on several rare spawns a while back. The answers were interesting to me, so I'll copy them here.
The Sro Ancient Cyclops:
There's one spawn in South Ro with a 30.66666666 minute repop time that has a 10% chance of being the ancient cyclops, but he has to spawn at night. He's on the same spawn table with a_desert_madman, a_dervish_cutthroat, a_sand_giant, and a_mummy.
Truth is stranger than fiction:
Pyzjn in Qeynos Hills:
There are two spawns in Qeynos Hills that can appear as Pyzjn. Their respawn time is 3 minutes.
These NPCs wander the zone. NPCs on these spawns are supposed to despawn at 7am / 9pm, presumably so they will respawn as the correct day or night version. Note that the night in EverQuest is 10 hours long and the day is 14 hours long, making it a little more difficult to find NPCs that only pop at night.
Vaniki, who drops the Willsapper, in Dragon Necropolis:
There are five spawns that can appear as Vaniki. The respawn time is 122 hours. Each spawn point has a 10% chance to spawn as Vaniki. Vaniki is unique, so there can only ever be one of them up at any given time. If the spawn is not Vaniki, it will appear as an unkillable invisible man placeholder. In either case, Vaniki or placeholder, the NPC will despawn 122 hours later.
This implementation makes the Vaniki NPC rare and essentially uncampable.
When these 5 spawns initially trigger (not when the zone comes up - there is a delay before they will appear) there's a ~41% chance that Vaniki will be in the zone. 122 hours later, if there is no interference, all 5 will despawn, and 122 hours later they will respawn. Each spawn location operates independently of the others, making the behavior of the system more complex if Vaniki is killed since the slain spawn will no longer occur synchronously with the other 4.