A common argument made by philosophical theorists who write about other animals is the so-called "argument from marginal cases." The argument is used in an attempt to show that other animals are either deserving or undeserving of moral status.
The argument is rooted in Aristotle's philosophy of classification, which places humans and other animals in a distinct hierarchy with privileged Athenian men on the top and other animals on the bottom – Athenian women and foreign, enslaved people fall somewhere in the middle. (Of course, this hierarchy still persists to a large degree in modern societies.) In terms of classification, the "argument from marginal cases" holds that a human animal is still a human even if they don't meet all the characteristics that are associated with humanness. That is, a human infant or mentally disabled human (the two most commonly discussed "marginal cases") may not meet the characteristics of rationality and intelligence associated with humanness, but they are both still humans. However, since these humans meet a lower standard or limited quality of humanness, they are considered "marginal cases."
The argument is easily used to promote overt speciesism by claiming that all human animals are humans and are therefore morally distinct from other animals. The argument sets the human species up as the moral standard. "Marginal cases" may meet a lower standard or limited quality of humanness, but they are nonetheless members of the human species and are therefore morally relevant.
Others attempt to use the argument to promote the moral status of other animals. Here the argument goes that if "marginal cases" are worthy of moral consideration even though they meet only a lower standard or limited quality of humanness, then it is wrong to exclude other animals from moral consideration simply because they too meet a lower standard or limited quality. Where in the previous situation the argument is used to argue that the human species is the basis of moral relevance, in this situation the argument is used to get around the human species as the basis of moral relevance.
However, regardless of whether the "argument from marginal cases" is used to advocate for or against the moral standing of other animals, it is a problematic and oppressive argument. In either case, the argument is founded on ableism and adultism. That is, the entire premise of the argument takes the marginalization of human infants and mentally disabled humans to be natural. The whole reason these humans are considered "marginal cases" is because they are assumed to be humans of a lower standard or limited quality, and the oppression of human infants and mentally disabled humans is considered a foregone conclusion. Since the "argument from marginal cases" is itself ableist and adultist, it follows that it would be unhelpful for challenging speciesism.
When philosophers use the argument in an attempt to show that other animals deserve moral status by making analogies to human infants and mentally disabled humans as "marginal cases," they are not only perpetuating adultism and ableism, but also speciesism. As "marginal cases," other animals are also considered beings of a lower standard and limited quality. What nonhuman animal advocates who use the "argument from marginal cases" are really saying is, in the words of George Orwell's Animal Farm, "All animals are equal, but some animals are more equal than others."