I looked at the first five entries in the Monster Manual, five consecutive ones from a random place in the middle, and the last five. Let's see what ecological niches they fill!
- Aboleth - Intelligent underground slugfish creature with psionic powers. Carnivorous. Evil.
- Achaierai - Giant intelligent flightless bird creature. Carnivorous. Evil.
- Allip - Undead wraithlike creature with mind-affecting powers. Does not eat. Evil.
- Animated Object - Stuff (e.g. tables, candelabras), but alive. Does not eat.
- Ankheg - Giant ant creature. Carnivorous.
- Formian - Giant ant creature. Diet not stated, but extremely aggressive, and possessed of psionic powers.
- Frost worm - Giant snowy worm creature with breath weapon. Carnivorous.
- Fungus (Shrieker and Violet Fungus) - Giant creature which is actually classified as a plant. Carnivorous.
- Gargoyle - Creature resembling living gargoyle. Does not need to eat, but attacks and devours "prey" anyway, for the fun of it, because is Evil.
- Genie (Janni, Djinni, and Efreeti) - Human-shaped creatures with magical powers. Seem not to eat. Come in Good, Neutral, and Evil.
- Xorn - Blobby mouth-with-arms creature. Scavengers.
- Yeth Hound - Flying hound. Carnivorous. Evil.
- Yrthak - Flying reptilian creature. Carnivorous.
- Yuan-ti - Snake-person. Diet not stated. Evil.
- Zombie - Undead. Do not eat.
Of a random selection of fifteen species in the D&D world:
- Seven are explicitly carnivorous. One species does not need to eat, but kills and eats other creatures anyway because it's Evil. (Capitalization not even mine.) One is a scavenger, three do not eat at all, and the others' diets are not stated.
- Many of them eat, enslave, and/or compete with humans and other humanoid creature, such as elves.
- Many are quite smart - as intelligent as the average human or elf or more so.
My very first thought is, do a lot of these carnivores eat each other? Or are there gajillions more antelope and things in this world than on Earth?
The worldbuilding concern that stands out to me most is the relationship between these species and humans (and other humanoid creatures). I myself have been recently working on creating a species that preys on humans, so I was already thinking about it.
The big thing I had to wrap my head around is that the likeliness of a species' existence in a world for any length of time is not just a matter of whether it would have evolved that way, but whether it would then last. Take a look at the real-life track record of animals that have competed with or threatened - even potentially - humans. You don't see a lot of Neanderthals around, and last time I checked, tigers were pretty darn endangered. We are a species that has caused extinctions for no real reason at all. (Remember the passenger pigeon?) Also, the D&D world includes not just humans but elves, dwarves, and halflings (among other things). If there are objectively, capital-E Evil creatures out there that prey on all of these species, you can bet that there will be pretty close to all-out war against those species.
Conversely, if these creatures are even more intelligent than humans/humanoids, and can themselves use tools, communicate complicated ideas, and possibly mind-control other species, how is it that vast populations of humanoids are still living free? I'm not saying this would be impossible, but you would want to give it some thought.
If you've got a species that regularly preys on humans - or on any other creature as organized and intelligent as humans - and there is no concerted effort to wipe these things out, you've got to know why. If there is such an effort, and that has so far failed, you have to be able to explain that, too. What are the predator's strong points? Speed, strength, magic traps? Does it live in the same areas as humans, or lurk in the jungle and eat anyone who comes through? Does it have other major sources of prey?
Of course, this isn't even getting into the fact that the world of D&D, like many worlds, includes multiple humanoid creatures that have no overwhelming competition issues. That's not just to say humans, elves, dwarves, and halflings, but centaurs, formians, goblins, yuan-ti, and several dozen other intelligent, organized social species with societies and large territories that all coexist. But I did not have a big enough breakfast to tackle that today.
Otherwise! In the realm of Good Points Made by Other Bloggers, here's one about when to make editorial changes, and one about adult relationships in YA lit.
Finally, via Nathan Bransford's blog, a hilarious video on book signings: