Outline of Peter Singer on Animals are Equal
We need to start eliminating. Nine of the beings are human, one is a horse. Can we quickly eliminate the horse on the grounds that it is a horse and not a human?Singer says NO. That is a violation of the principle of equality. Since obviously the nine beings not chosen are likely to die, and to die a painful death, we have to assume that the ten beings all can suffer pain. The non-speciesist version of the principle of equality would say that when suffering pain is involved we must give equal consideration to the non-human animals.
BBC - Ethics - Animal ethics: The ethics of speciesism
Peter Singer: Speciesism is an attitude of bias against a being because of the species to which it belongs. Typically, humans show speciesism when they give less weight to the interests of nonhuman animals than they give to the similar interests of human beings. Note the requirement that the interests in question be “similar.” It’s not speciesism to say that normal humans have an interest in continuing to live that is different from the interests that nonhuman animals have. One might, for instance, argue that a being with the ability to think of itself as existing over time, and therefore to plan its life, and to work for future achievements, has a greater interest in continuing to live than a being who lacks such capacities.
Since many decisions we make involve allowing or requiring pain, it is important to recognize that all animals are capable of suffering pain, and that pain, as pain, is of equal moral worth to all sentient creatures. Thus species ALONE cannot be a relevant principle of moral decision making. The general principle of equality, in relation to pain, must be extended to all animals.