Animal feed, fertilizer.
The problem is, people love horses, but what do you do with an animal that isn't suited as a pet or trail horse for people? Not many people are willing to take on the non-trivial expense and duty of equine care for a beast the kicks, bites, and can't be ridden...
And then, there is the question, of what do you do with an animal that is overage or infirm? People will pay -- grudgingly, and just barely -- for elder care for their parents. They won't do it for their horses. The horse winds up at auction, and as the young lady points out in the article, one or more of the buyers is assessing these animals, not from the viewpoint of a horse lover, but as livestock. There it is.
A horse requires a lot of space and a lot of care, even if it's, well, "healthy as a horse."
Of course, no one who wants to maximize his profit from a horse he intends to slaughter will fail to feed and water the beast. That some people are cruel or incompetent at treating their animals is a separate issue than the fact that some animals are sent to slaughter. (I think gratuitous mistreatment of animals is far, far, worse than simply killing them for food or products... you might well disagree).
Here are some of the products that come from slaughtered horses:
In the USA a lot of horse meat goes to soup kitchens and other programs for the poor.
d.o.l.
Criminal Number 18F
If only it really happened like that you could put it in a Hallmark card. But it doesn't. My horse will be put down here at home with tears and a needle. Not shoved down a chute not comphrehending why his comfortable life has suddenly turned into a nightmare by people he will still try to trust.
I have no trouble with the rendered uses of his body if I have a truck pick him up. I may not want to watch... there is no dignified way to move a 1200 pound animal, but it is the mixing of live slaughtered horses and the disposed dead that keeps me from knowingly buying the products I can avoid, such as pet food.