So I've been doing some thinking! Since I've been updating my fursona and her alternate form, I decided to try to figure out a way to tie the two forms together, and here's what i came up with:

Rinny is a lover of magic, and found some old looking, dusty tomes in an occult library, and without her knowledge they had in them the binding ritual for a demon of lust. At home she inadvertently performed the ritual with herself as the vessel, summoning the demon and binding it to herself. It quickly corrupted her and made her part demon, needing to feed on sexual desire or else she fully transforms and loses control until she feeds (maybe with time she learns to harness her power and transform at will :3)
Even after turning back to her normal form, some visual aspects remain, like vestiges of horns on her forehead and behind her ears, her sclera turning black, and a binding sigil on her back

I'm still working on the details and most of that is like, literally off the top of my head, but soon i'll have it more fleshed out and a few pics about it :3

I also updated the refsheets I posted recently with these new visual changes:
https://www.furaffinity.net/view/57285606/
https://www.furaffinity.net/view/57285622/
https://www.furaffinity.net/view/57388508/