Just as with my favorite monster, I don’t have a least favorite. My issue was with the so-called “splat” books released for D&D. Frankly, there have just been to many of them. Go and read through some of the expansion books for the Monstrous Manual. They’re dreadful.
This is, of course, a generalization. In every D&D book is at least some small kernel of wisdom, but the more books they tried to pump out, the more the quality deteriorated. There numerous examples of this, but one has always stuck out to me as really just not trying on the part of the writers.
There is the classic “Nightmare”. A very cool monster. An infernal beast with flaming hooves and mane. I’ve never disliked this monster. In fact, I don’t dislike the more powerful version of a Nightmare. I do however, dislike the creature that is directly related to the hellish steed, the Cauchemar.
Because Cauchemar is French for “Nightmare”. There was basically zero effort in making up that name. I understand wanting to have a more powerful version of the more well-known Nightmare, but just looking for the exact same word in a French-English dictionary is just truly uninspired. At least change the spelling a little bit, right?
This problem exists in a lit of the splat books. Lots of cool concepts ended up as boring garbage. At least since it’s gaming, taking the content and remaking it into your own thing is strongly encouraged. In the world of D&D though, sometimes less is more really makes a difference.