is there some sort of in-universe slur that would be used against preternaturals?
Not specifically. Fic writers in the past have had prejudiced characters use "preter" pejoratively to describe preternatural beings in general. On the other hand, fics have also had preternatural beings use the same word to describe themselves - Preter Rights is an advocation, for example.
Going off of that, it might be a word caught in a kind of cultural tug of war between those who use it as an insult and those who use it as an identifier - neither good nor bad in itself, with any connotations coming from the tone of voice with which it's said or the context in which it's used.











