Monster cup
From Arkham Horror Wiki
The monster cup is a container that is used to hold monster tokens that are not in play. During the initial setup of the game players choose a bag, bowl, or other opaque container to be the monster cup.
Mask monsters are not added to the cup during initial setup, unless the Ancient One is Nyarlathotep
Whenever a player is directed to draw a random monster, a token is randomly picked from the monster cup.
Discarded monster tokens are always returned to the cup unless otherwise specified.