Monsters are enemies found nearly everywhere in everybody's Fanon and the canon Miitopia.
They are typically the antagonists of the game and usually hold Mii parts captive, for varying reasons.
Monster Lists in Fanons
Monsters in Fiend's Call (JR's Fanon)
Harsh Edition's Monsters/Bosses (CR37's Fanon)
List of monsters (miitop) (Doge2094's Fanon)