The foundation of animal-human romances in modern Japanese media lies in traditional folklore, specifically through the concept of 異類婚姻譚 ( iruikonintan ), which translates to "tales of marriages between different species."

From Fox Wives to Capybara Couples: Exploring Animal-Human & Animal-Animal Romance in Japanese Storytelling

If you'd like to narrow this down, let me know if you want to focus on: Specific (like the Kitsune or Yuki-onna)

In the real world, this manifests in Japan's famous "pet mourning" rituals. Unlike the West, where pets are "members of the family," in Shinto-influenced Japan, a deeply beloved pet can be enshrined as a tsukumogami (a tool with a spirit) or even a minor deity. Elderly Japanese couples who have lost their children sometimes speak of their dog or cat as koibito (lover)—not in a carnal sense, but as the sole recipient of their remaining emotional devotion.