"Animal rights" is its own can of worms, but seeing as we have a huge meat-for-food industry around the world, most people (right or wrong) clearly think that animals are not on moral par with human beings. We eat them, after all.
So, aside from objections regarding the animal's rights, and given that it would work, I don't see any moral problem with xenotransplantation (animal-to-human organ transplant). I understand the discomfort about human-animal chimeras, but just as we don't become the person who donates his or her organ to us, we won't become pigs if we happen to have a pig's kidney. For example.