They would have to be able to hold a conversation. Octopi are much, much smarter than cows, I will still happily eat them... though I feel a little guilty about that. Cows, though? No guilt.
That said, there is much tastier red meat to be had, if only we diversified our livestock. Kangaroo and Ostrich, in particular. And both of those animals are cold-blooded killers who deserve to be eaten.