Does it matter if it was?
Or did the menu specifically state that it was beef?
If the menu just said "steak" there's no reason why it shouldn't have been horse.
Reminds me of a time I had a stand selling venison at a food festival. Now, bear in mind that I has banners up with pictures of deer, I had some deerskins and antlers on display, the word "venison" is on all my promotional and display material and, what's more, every packet of meat says "Welsh Venison" on the label in large black capital letters. No room for confusion, you would have thought. And one lady spent ages looking at all the packs on display, and then asked me which one was the venison? They're all venison, I told her. Oh, she said, so why do the labels say steak?
