I saw someone suggest this on another wiki page, possibly one of the ones for Demon's Spear - prompted me to look up the fight to remind myself. The Demon's Spear definitely looks like it could come from Demon in Pain, you can see a few parts where the unique crabclaw shape of the Spear tip might have come from. You can see a lot of good shots of him in this video liike
https://www.youtube.com/watch?v=_02sH8E6RGU - around 4:30 you can see his arms pretty clearly which I suspect to be most likely where the Demon's Spear's materials came from - either the extra appendages extending upward from his fingers, or maybe the bone that forms his forearms, extending from the thumb and forefinger.
For anyone wondering, from the Demon's Spear Description in DS1 - "Carved from the bones of fellow demons. Wielded by the slim lesser Batwing demons. The weapon of these chaos demons wandering Anor Londo are different from those of other chaos demons, and are imbued with lightning."
This implies the Batwing Demons were enticed to betray their own, most likely by Gwyn and his ilk, who crafted weapons for them using his lightning and the bones of their brethren. People have wondered for a long time which demon the spear is crafted from, as the source wasn't obvious all through DS1 & 2. I very much like the fit of Demon in Pain personally.