It's a protein acquired from animal hides and bones. (sciencedaily.com)
(Most animals hide pain and discomfort very well, so even if they don't seem to be in discomfort doesn't mean they are not in pain or are not suffering). (leesburgvetblog.com)
For armor, you'll need specific items like animal hides, iron, wood, and bronze. (thekoalition.com)