Obviously there is no requirement for companies to list "horse" in the ingredients of their fertilisers. Check the packaging, and if it says anything about meat meal, or animal byproducts, then you can't rule out the possibility that it may contain horse.
On one hand, at least there is minimal wastage when a horse is slaughtered, as parts of the horse which cannot be used for other things, such as pet food, can at least be rendered into other products with a useful purpose. It should also be acknowledged that some horses sent to knackeries are already deceased on arrival. Their owners have either chosen to dispose of the body this way, or have perhaps not had any other choice, as burying or cremating a horse can be difficult both financially, and in a practical sense.
The aim of the exercise here is to provide information about what products may contain ingredients that were once part of a horse. What people choose to do with the information is obviously up to them.
If you personally are not keen on the idea of putting bits of ground up horse on your garden, then there are plenty of other fertilisers out there that you can use instead.
Plain old horse manure perhaps?