One other way of saying "melt that horse" is "cook that horse." I forget the exact recipe for hide glue, but I think it mostly is a process that involves soaking and heating hide with lime (not the fruit). Also materials that can be used in the production of leather. Anyways, if you cook enough horse and you make enough horse leather in an area with lime, and you're observant enough, hide glue is sort of an inevitable discovery.