I haven't read all of the posts in this thread yet because it is so long and I may be repeating something someone else has already said, but here is my take on this.
Judging by your avatar and your description of the main character, I would say you are either a fan of anime, manga, doujinshi, hentai, or all of the above. There is nothing wrong with including aspects of Japanese culture in your writing. I do it as well with Japanese names for characters and locations, but you have to consider the manner in which you wish to include such aspects. Including your horse-man character in an erotic fantasy novel will help to add to the assumption made by many that Japanese culture is perverted, which is only true of certain people in Japan and not the culture as a whole, and may help to detour your readers from Japanese culture. If you are okay with that then you should not worry about it, but if you want to promote Japanese culture I would not recommend putting such a character in your book.
Secondly, putting a character like this into your book may seriously damage sales. The question on my mind is why are you adding this character? Is your novel meant to be a psychological erotic fantasy in which readers are meant to see this character as perverse and other characters in the book shun him or try to change him? If the story revolves around a psychological journey this character undertakes from changing his perversion over to a "normal" view of sexual contact with human females (seeing as he is more human than anything else) then there may actually be a desire for people to read the book, but if he does not go through some sort of psychological change and continues to indulge in bestiality throughout the book you will be lucky to sell 10 copies of the book.
