[JohnRTroy]

Well, trademarks are more or less limited to what Wizards chooses to register to the US Patent and Trademark Office. They have a few terms trademarked, such as Dungeon Master, etc. But there are not many federally registered trademarks.

Technically, created creatures come under copyright, not trademark. The name Mind Flayer is not a trademark, but because that's descriptive and not part of the game, they can say you are violating their copyright of property if you write up a description of the mind flayer that is similar. If you wanted to have a similar styled creature, you could, but it can't be too similar and you shouldn't use the name Mind Flayer.

This all depends on how unique the creature or creation is. The more Wizards can prove that the existence started with its own writers, the more protection they have. Dragons are from loads of fiction, but though the Drow are named after other fictions, the "spider-goddess-worshiping elves who live underground with a matriarchal society" would be off limits. Vampires are from mythology and Stoker, but the Warforged are from Eberron's creators and thus property of Wizards.

You can get around it by making sure your creation is different enough. There are a lot of Superman Pastiches (Gladiator, Hyperion, Supreme, Mr. Majestic, The Sentry) but each is significantly different. It depends on your purpose. I would suggest that if the attempt is to simply "file off the serial numbers", it's a weak protection, but if you want to use the idea while putting your own spin on things, that's more likely to be protected.

ETA--I assume monsters in a world are copyright just like individual characters are. For instance, I remember reading Peanuts books and remember reading that Peanuts characters are copyright on the day they were created, Charlie Brown in 1950, Woodstock in 1971. While I don't know how it works for a gaming world, you can assume that monster description and names (when unique) are protected by copyright.