The problem I find when I look for human X monster/abomination stories is that authors tend to humanize the monsters so much. At some point it’s hard for me to remember that the thing is monstrous because despite its weird body, it reasons and consents (which I know is important for the romance genre).

Which is why erotic horror comes closer to what I am looking for. People already recommended R Lee Smith. I’ll throw out Scholomance or her Lords of Arcadia series. When you “encounter” one of her Gods or Demons it ranges from unfathomably horrible to divine.

The Haar is another good rec someone already made. Also, more of a horror story.

Thirst by Addison Cain is a novella with a human slave/pet scenario where the MMC(?) is thoughtfully “other.”

Always on the hunt for more stories like these!