It's a weird one, isn't it? It was fine to eat before we knew about the horsemeat situation, and it's not a health risk, as far as we know.
It just feels different now. I know I couldn't eat it now that I know. A little knowledge is a dangerous thing, as my grandma would have said. She would also have said what you don't know can't hurt you!
I'd bin it. But I have eaten horsemeat in France. Go figure. I think it is probably some ropey old horse, as horsemeat is more expensive than beef usually, I'm told.