Earlier quoted context omitted.
Animals that have to be euthenised for one reason or another cannot, and nor should they enter the food chain for humans, cats, dog, or otherwise. Firstly, there are the drugs they use to humanely put down the patient which would be toxic to down stream consumers of the meat products, and secondly there maybe concerns about zoonosis.
If they choose to put down a horse and decide to used for human consumption, then they will certainly use a different method ( shot, hammer, etc ).
This in part was why the horse meat scandle was such a problem in the UK.