When I play Abhoth (from Under the Pyramids expansion) as the Ancient One, his Setup section says:
Set aside 8 Cultist Monsters and all Abhoth Special Encounters.
Then the Reckoning effect for this Ancient One states
The Lead Investigator spawns 1 Cultist Monster on a Wilderness space that does not contain a Cultist Monster. Then, if there are 6 or more Cultist Monsters on the game board, advance Doom by 1.
Cultist fights are special for this Ancient One, you do not have normal Combat Encounter, but as described on the Ancient One sheet:
Whenever an investigator encounters a Cultist Monster, he draws and resolves a Spawn of Abhoth Special Encounter.
The Spawn of Abhoth Special Encounter cards do not specify what happens with the cultist card/token, you can just win/lose the fight.
When I played, I assumed that I spawn the Cultist from the Ancient One sheet, from these 8 Cultists I set aside.
What should I do when I defeat the cultist? Should I put it back on the Ancient One sheet, or should I put him to the normal monster pool, as with any other non-epic monster? (or some combination of both.. should I put the cultist back on AO, but only to the original count of 8, and then put the cultist to the normal monster pool..)
When I played, I did not think about it much, and just put them back on the Ancient One sheet. This could result in having more than 8 Cultists on the sheet, since you can draw Cultist Monster from the monster cup when you have Monster surge or open a gate (although the chance is lowered because 8 of them is already set aside).
But I started having doubts when the Ancient One awakened. Then you have to spawn all cultists from this sheet except 3, and then whenever Doom would advance, you spawn another cultist from the sheet. When there is no cultist left on the sheet, investigators lose the game. This condition is the one that made me doubt my decision to put them back on the sheet, since generally, in Eldritch horror, the rules are against the players, to make the game harder, so I started to think that I might be doing it wrong.
I did not find any mention about this in the rules for the expansion.