I'm not a lawyer but I play one on the internet. For the purpose of this discussion, intellectual property breaks down into two concepts that are often confused: copyright and trademark.
Copyright is the right to print copies of a specific work. You can, to a limited degree, copyright a character outside the context of a particular work, but that gets very complicated very fast in the sense of "how much difference is enough?"
Generally, copyright in the context of tabletop gaming is extremely limited. You can't copyright game mechanics -- nobody can own the concept of "+4 to hit" or "AC 12", nor the concepts of Feats or Classes. The rules as a conceptual thing cannot be owned. You can, however, copyright a specific expression of the rules. So you can write a game that functions the same as D&D using your own words and expressions, and that is legal, but you can't use the exact text WotC has printed and claim it as your own separate game (and again this can get complicated because tweaking a word here or there isn't enough to count as a new work).
To some extent, WotC can claim a copyright (or possibly trademark) over certain game element names like "Tiefling" that don't have a particular mythological basis (the term appears to have been invented by WotC or TSR), but that copyright does not extend to the concept that the name represents. That is to say, you may not be able to write up a section on Tieflings, but you could call them Nephilim and have virtually the same species, because the concept of a half-demon child is not unique to Wizards, and they simply can't own a story idea like that even if it weren't a preexisting idea.
But I mentioned trademarks, and that's the broader and even more complicated side of IP. If copyright law is complex, trademark law is a portal to the Far Realm. A trademark is anything that is identifiable as belonging to a specific company, from a specific character to a setting to a particular typeface. Trademark protection is aimed specifically at preventing confusion among consumers -- it's basically anything that, if an average person involved in that particular hobby or industry sees it, will make that person say "Ah, this is a product from Brand X!"
In short, trademark represents anything Wizards can claim as "brand identity". I know for sure they've made that claim on some specific monsters including, as I recall, the beholder, owlbear, mind flayer, bulette, and flumph. Maybe gnolls, too, at least in the "humanoid hyena" form (the word "gnoll" predates D&D, but only as a sort of general term for some kind of dangerous fae-like creature). Much of D&D is based on existing mythology, folklore, and older stories and is thus blocked from any kind of trademarking -- for instance, they can't claim dragons are brand identity for Dungeons & Dragons because you can easily point to older works featuring dragons.
But there is a point where another work can't claim they're drawing on older works because they've used too many details specific to D&D. For example, if you have blue dragons that spit lightning, specifically, in your game, then you might be on unstable ground because associating specific color-coded dragons with specific breath weapons could possibly be a kind of brand identity.
Similarly, if you have kobolds who are tiny dragon-people, you might have a problem because while a kobold is a traditional germanic faerie creature that lives under the earth and sets traps for the miners, the tradition does not specify them as little reptilian humanoids.
But I say "might" a lot here, because trademark is hideously complicated. Much of it runs on case-law and "reasonable person" standards that don't lend themselves to clear bright-line rules. Even big corporations often have to actually run through a lawsuit to determine if something is or is not trademark infringement, and sometimes the outcome just depends on the individuals involved in the decision.
Which is all to say that it likely isn't possible to draw a clear border around what constitutes WotC IP and what doesn't. (Hint: Corporations like it that way. They'd prefer that you stay far away from their territory.)
If you want to be cautious and avoid infringement, your best bet is to make things up yourself or use only elements you can specifically trace to pre-1970s literary roots or are in widespread use in non-WotC products. For example, green-skinned goblins and orcs with jutting lower teeth probably originated with a specific line of wargaming miniatures intended for use with D&D, but since it's widely used trope in many different products (such as Warcraft and Warhammer), you're probably safe doing the same. In trademark law, "everybody is doing it" is actually a legitimate defense, since you can't claim "this element automatically implies it's from my company" if it's in common usage in your particular industry.
Note that "trademark" and "registered trademark" are not the same thing. A company doesn't have to register everything that could possibly be considered trademarked material in order to get legal protection on it. A registration is a sort of pre-determination that a given element is theirs and theirs alone, but it defines the minimum area they have trademarked, not the maximum.
I'm sure this isn't as useful as you could have hoped, but that's just the way IP law is. And I'm sure I've missed some points or said some things that aren't quite accurate. As said, I'm an interested observer, not a trained lawyer.
