Muism has no canon and no fixed pantheon, and the population of spirits a mudang deals with is drawn from wherever Korean history has left figures with power.
At the top sits a sky power, Hananim or Haneullim, distant and rarely petitioned directly, a name later taken over by Korean Christianity for its own God, which has caused lasting terminological confusion. Below and much more active are the spirits actually addressed in ritual: household gods with defined stations, including the house lord in the main beam, the birth grandmother Samsin who is responsible for conception and children, the kitchen god, and the earth spirit of the site; mountain spirits, sansin, whose small shrines stand behind many Buddhist temples; dragon kings of sea and river, important in fishing villages; and gods of disease and smallpox.
A large category comes from history. Generals, kings and figures who died violently or with grievance can become powerful spirits, and are among the most commonly invoked; the General and the Boy and other stock figures appear in the standard segments of a gut. Ancestors appear too, and here Muism sits alongside Confucian ancestral rites rather than replacing them: the Confucian rite honours the correctly settled ancestor, while the gut deals with the one who is not settled.
That last point explains the emotional core of much Korean ritual. Han, a hard-to-translate sense of accumulated unresolved grievance, and the associated idea that the dead who died wrongly or unmarried or without descendants remain stuck, generate a specific ritual response: the mudang gives the dead a voice, lets the grievance be spoken to the living, and unties it. The jinogigut performed for the newly dead in the Seoul tradition is built entirely around that.
The system is expandable. New spirits enter as new powers appear, and mudang in the twentieth century added figures from recent history to their repertoires.