Monster Classes are special, in that, you will not be able to create them in the Dark Assembly until you defeat them at least once in the game. Note that since Item World is completely random, it is possible to encounter them much earlier than their first normal appearance. The Prinny is the only Monster Class unlocked from the beginning of the game.
As with Humanoid Classes, Monster Classes have a total of six tiers and function similar to Humanoids. In order to unlock the next tier for a Monster Class, you must level the previous tier to a specific level.
