The Q
Hoop’s Villain, Reality’s Hero
They own the name "Bray Wyatt", "The Fiend", the likeness of The Fiend (ie he can't use a mask or facepaint that looks like The Fiend), and some other stuff related to the gimmicks ("Firefly Funhouse", "Let me in", etc). But if he wanted to show up anywhere else playing a cult like figure named Windham Wyatt, nothing is stopping him. It's the same deal with Malakai Black. The imagery and tone of the character is the same. The biggest difference is the first name.
i don't think WWE ever trademarked let me in. Or at least they hadn't as of a while ago. Before they cut him lol