That gallery is actually quite out of date, and doesn't contain any of the entries I myself contributed; you can see all the material here if you're interested:
https://booru.org/ntrhaven/index.php?page=post&s=list&tags=ntr_encyclopedia
And I understand exactly where you're coming from, fantasy is such a bizarrely underused genre for NTR eroge despite being so (understandably) prevalent in RPGmaker.
Never guessed there would be a NTRbooru out there lol, thanks a bunch for the link and nice works ^^b.
Going back to the NTR encyclopedia, what caught my attention the most is the "Living Doll", her entry could work best for a mystery NTR, where it develops on human psychology, with the MC, after recently discovering the fetish that is NTR, is in denial stage, and would reject any thought of sharing his lover, but still wishes for it deep down. Well, this is mostly from the viewpoint of a fan of Netorase which is why her description caught my attention the most besides the "Mermaid". Regarding the latter, I may have the privilege of becoming aqua man (lol xD) if I were that mermaid's boyfriend since I'd accept her for who she is and would want her to keep cheating but with my consent (or not for mystery's sake) due to my Netorase fetish.
What I like the most about the "Living Doll" and "Mermaid" is the fact that they sincerely keep loving their lovers and stay loyal to them, I mean, this might sound weird for most NTR fans, but personally, the NTR is more spicy when there's love between the MC and the heroine from beginning to end (and the moment it is lost, the NTR is no longer prevalent, much like how the Mermaid no longer have interest in cuckolding if she loses the love of her life as described on her entry), but the heroine would still find sex with others more enjoyable. I'm not sure how to well explain it but, take "Marina's Cuckolding Report"
https://vndb.org/v12949 as an example and you may probably see my point. Although as I said, this is all my personal preferences and are merely my opinions, every persons got their own way of enjoying their fetish.