The horse is a danger zone in ways that some other genre or category elements are, and some are not. Two characters can fall in love without anybody insisting that the story as a whole behave as a functioning member of the Romance genre. But if they have extremely explicit sex on most pages of the story, or enough of it early on despite how the ending goes, the story will most likely be parsed as Erotica or Pornography of some sort. Just having a dead body show up mysteriously in the first chapter does not make a book a Mystery -- you have to devote most of the structure of the book for that -- but if you have a children's book whose main character is on a sports team that shows up in the story at all, you may well be writing a Children's Sports Book whether you meant to or not. Some elements are category black holes. (Black holes, for example.)
So despite the presence of horses in this chapter -- and despite the presence of horses in Dwarf's Blood Mead -- I am very carefully avoiding anything that might smack of Horse Book nature. I am handling the horses very differently because I am conscious of Horse Book devotees and their obsessive ways; some of those devotees were my best friends when I was 11, and they urged their Horse Books upon me. I don't want to write a failed Horse Book. I want to write something else completely. The analogy with smut writing goes further: I am willing to believe that someone could write a book that was interesting to me that was a Horse Book, just as I'm willing to believe that books primarily about sex, sex, and more sex could be interesting and well-done. I nominate Someone Not Me to write them, though.
Hmm. I'm thinking of other children's book categories, and I'm really not opposed to writing a School Book (although I would prefer to wait until people had a little more distance from the Harry Potter phenomenon and could realize that J.K. Rowling did not invent the School Book!) or a Club Book or a Kid Detective Book. I would have sworn up and down that I would never write an American Pioneers Book, but the utopian commune book that's simmering in the back of my head does share many of the features, so maybe I'm wrong. We'll see.
People who are writing: are there any categories you're staying well clear of? Any categories that are trying to impinge?