I have to wonder how the intellectual property shakes out with Beholders and Mindflayers and Kuo-Toa and so on that WotC has trademarked. I think if you're doing business on DM's Guild, I think they let you do more than if you're not, but I haven't found any indication that MZ4250 is doing business on DM's Guild.
I once knew an author who wrote a book with a character whose name was the same as a D&D character (I don't remember which one... it wasn't "Vecna" or some super-well-known character name, though). Aside from the name, it was a completely unrelated character, and the name collision was entirely coincidental. He got a cease-and-desist from WotC. And he didn't have the means to fight anything, so he quickly unpublished the book and hoped they'd leave him alone. (They did, AFAIK.) At least he didn't get harassed by the Pinkertons.
But yeah. WotC is basically Satan, and not in a cool way. (Oh, sorry. I misspelled "Hasbro".)