Sheep don't have anything like free will, because free will needs abstract thinking, thinking about the future, possibilities. Animal is always now and here, and reacts to the envirement according to instincts and previously learned reflexes. Animals in slaughterhouse can escape as well, we are just better predators then wolfs so it happens rarely. It's not a point anyway. And slaves were people and possessed reason, therefore slavery always was and will be violation of natural rights of the people. Nothing comparable to animal breeding.
RE: Vegan Anarchy: Do animals have the right to life, liberty & the pursuit of happiness?