Cecil, you're pretty much the man. I think I got one for you. In the old westerns and Bugs Bunny cartoons that pretty much formed my thought processes as a child, they would always threaten to send the old horse to the glue factory. It's mostly used as a figure of speech these days, but yes, horses typically were sent to the glue factory or rendering plant back in the days when they were mostly used as work animals.
Roses are red, violets are blue.
Horses that lose are made into glue. Rendering plants are the recycling links in the food chain. They take fat and bone trimmings from grocery stores, waste scraps from restaurants, and dead animals. They cook the meat and fat products together and grind it up. But as I say, the rendering plant isn't the only possible destination for a horse that's outlived its usefulness.