omegaverse
A sexual setting loosely based on wolf behavior, commonly associated with yaoi stories.
Humans are split into one of three biological categories: Alpha (Dominant species, may have actual wolf biology), Beta (Subservient species, humans as we know them) and Omega (Lowest rung, both genders are capable of getting pregnant and can go into heat).
While this setting originated in fanfiction, it has expanded to other mediums like visual novels and manga.