When I worked at a wildlife facility, we had some gentlemen who would deliver chicken carcasses from the local Perdue plant (an entire truck bed). They kept a bucket in the back of their truck, and would toss in random bits of fat and skin that plopped out here and there. The bucket would ride around summer and winter, and would disappear every 1–2 weeks. I finally asked them what happened to the noxious, smelly mess and they told me it was sold for makeup and soap making.
Since that time, I have not used common commercial soaps. The idea of rubbing rotten animal fat over my body does not appeal. So I use soaps with plant based fats.