Eepy evenings
Just in case anyone was wondering. Trademarking fan-made characters and giving credit to the fan creators when it comes to fanfic, is only necessary when you are writing within that can creators AU.
Siren from bonesandthebees’s Clinic series, is a really good example of this. If you want to write within the clinic series, it is a helpful resource for your readers who may be unfamiliar with clinic, if you were to tag the OG creator of that niche Au, with tons of worldbuilding already established in the content. You wouldn’t tag bonesandthebees for creating an superhero/supervillians sbi au, that au is so vast and versatile. You could thank them for inspiration if that occurred, but still.
Like hypothetically: if someone wrote a witchbur fic, that didn’t focus on my own preestablished AU— like it was simply mashing the two items together, Wilbur and the concept of engaging in witchcraft—that’s not something you need to credit me for. You can thank me for the idea, if that was how you came to that idea, but it’s not necessary.
So. TLDR? If you are writing for someone else’s au—give credit to the author. If its just based of the vague concept of the Au? Not necessary.