Traditionally understood, such transformations are a metaphor for humanity's uncomfortable relationship with our animal nature. Vampires, for example, represent the perceived destructive power of sexuality. Dracula, the first vampire, seduces a woman, destroys her life and damns her soul with sex. She, in turn, also becomes sexually liberated, and attempts to seduce others into her lifestyle. Camilla, the next famous vampire, seduces a young woman into homosexuality, using the same underlying dynamic. A hundred years later, the Twilight saga used these elements to describe two teenagers trying to practice sexual abstinence. Action hero half-vampire Blade plays out a Jesus-like figure - valiantly resisting temptation (his bloodlust) to protect and redeem humanity by acting selflessly.
Vampires, and werewolves, describe both the seductive power of sexuality and the monstrous way we have been taught to perceive it. Mermaids similarly play on lust - in this case, the lust of sailors at sea, desperate to find women. Mermaids are beautiful but forever inaccessible and unknowable, a living metaphor for the feminine mystique.
Ogres are my favorite demi-human transformation. In Shrek, Princess Fiona becomes an Ogre as a way of teaching a lesson about beauty, love and acceptance.
Some demi-human stories utilize do not have an explicit transformation scene. For example, the story "Cindi" relies on the character (and the audience) realizing the true nature of the demi-human rather than actually showing them changing.
In the Demi-Human section of my stories, you'll find two demi-human tales: one about ogres and the other about mermaids. I'm going to be adding another ogre-related story soon, the result of a very long and detailed story swap. It's been the product of many months of work.
In the meantime, here is a short story that I recently finished. It contains some transformation elements but they are not the focus. I wrote it for a prompt that read only "love between rocks." The title - Stone Age.