Time was when horsemeat was accepted as being for pet food. I haven't had a cat for 25 years and was quite suprised at the stuff we pamper our pets with now - Brooke is currently feasting on Felix Lamb with game jelly, Duck with spinach, Trout with spinach and Mackeral with tomato. On the face of it she eats better than me
However looking at the breakdown its only 4% lamb etc so I'd expect the rest to be random meat including horse. Frankly I'm cool with that so long as its not going to poison her.
Having learned about pet food whilst working in a pet shop for 9 years, a vegetarian can eat 80% of what's in cheap tins and foils. I believe a man working for Whiskas ate a can of pet food on tv some years ago.