Every creature has a creature type. Aberration, Beast, Celestial, Construct, Dragon, Elemental, Fey, Fiend, Giant, Humanoid, Monstrosity, Ooze, Plant, and Undead. Nothing exists which can not be categorized. Whether or not the player is aware of the creatures type is another matter.
Likewise magic can be divided into spells and curses. This is essential if we are going to follow the rules explained in the supremacy of magic lore article. if its a spell then dispel magic should dispel it. If it is a curse then remove curse should remove it.
At no point should some uncategorizable creature or magical effect be encountered. This rule is essential to maintaining the hierarchy explained in the ranking of adventurers lore article. It is essential to the functioning of this world that high ranking adventurers should be able to handle low ranked challenges with ease. This is because they have access to abilities with very consequential effects. If the DM were to create made up category of creature or spell then this breaks the whole idea of supremacy of spells because there would be no spells or abilities in existence who's descriptions were written with a way to counter that effect, so even the invincible S-ranked adventurers would have no way to solve such a problem.
Note that the Frans has been given explicit permission to create custom characters so long as they conform to the above rules. However no such permission has been enabled to create custom spells. Any spells encountered in this world must therefore be a spell that already exists.