This is only the tip of the iceberg, I expect. Processed meals invite suppliers to put "any old shit" into food and conceal it with flavourings, and it isn't surprising that horse has shown up. I expect that we've been eating horse for decades.
The answer is to retreat from foods where you can't identify the contents, to "meat and two veg" combinations (or make your own food from raw ingredients) where you have a clearer idea what's on your plate.
