So I was reminded by the buzz around @VaatiVidya's latest video of one of my own lore theories that I don't think I ever detailed here on Twitter. Let's lay the framework:
Throughout the series, we've been shown that are apparently four primary ways to create a demon.

One, you have demons that were born directly from the Chaos Flame, such as Demon Firesage.
Two, you have beings that were bodily transformed into demons, such as Quelaag.
Three, you have beings that were warped into demons by their desires, such as Covetous or the Corvians.

And fourth, most importantly for this theory, you have demons that were spawned by magical items left unattended for a long period of time. The Titanite Demons are the remnants of ancient Titanite Slabs, and Centipede Demon was created from the magic of Ceaseless's lost ring.

In Dark Souls 3, there's a Stray Demon on top of the bridge above Farron. Near that demon, you can find the corpse of one of Havel's knights, all of whom wore magic rings that helped them bear the burden of their immensely heavy armor. Importantly, the body is missing its ring.

Your first thought might be that the demon caused the death of this knight, but I think the reverse is true. The demon's soul transposes into Havel's Ring, suggesting to me that the knight died on that bridge and, left alone for untold ages, their ring spawned a demon of stone.

Vaati mentions in his video that part of the appeal of Dark Souls lore theories is finding details that line up in such a way that you're certain there's a deeper story being told, even if it can't be explicitly confirmed. The Havel's Ring theory feels like one of those moments.