x is similar to y (where the similarity depends for its strength upon some assumption which begs the question).
Do animals deserve the same respect as black people? That's the question posed in an online exhibit by People for the Ethical Treatment of Animals. The exhibit pairs a slave auction with a cattle auction, two hanging black men with a hanging steer, herded Native Americans with herded cattle, a burning black man with a burning chicken, a shackled black ankle with a chained elephant hoof, and a pygmy in a zoo with a monkey in a dress.
Source: William Saletan, "KKK vs. KFC", Slate, 8/17/2005
An analogical argument begs the question when the strength of the analogy depends upon some controversial point at issue.
Some animal rights supporters draw a detailed analogy between the treatment of nonhuman animals in contemporary America and the Jewish Holocaust of World War II. Slaughterhouses are compared to Nazi extermination camps, both of which involve large-scale killing through an efficient, assembly-line process. The conclusion is that the treatment of animals is morally similar to the Holocaust perpetrated by the Nazis.
This argument begs an important question concerning the moral status of nonhuman animals. Of course, many advocates of animal rights and ethical vegetarianism consider all animals to be morally equal, so for them this is not a disanalogy. For the audience at whom such arguments are directed, however, there is a moral difference between human beings and other animals. In fact, much of the moral outrage at the Nazi atrocities comes from the treatment of Jews "like animals", as we say, violating their human dignity. Thus, without the underlying assumption of the moral equality of humans and other animals, the analogy between the Holocaust and so-called "factory farming" is superficial. If it is not wrong to kill animals for food, then it is not wrong to do so efficiently and in large numbers.
This is not to deny that there is inhumane treatment of animals; rather, that there is a moral equivalence between such inhumanity and the inhuman treatment of persons in Auschwitz. Furthermore, if all animals are indeed morally equal, then killing animals for food is equivalent to murder. However, this claim must be shown by a separate argument, and cannot be established by the tendentious analogy with the Holocaust, which depends logically upon it.
David Hackett Fischer, Historians' Fallacies: Toward a Logic of Historical Thought, (Harper & Row, 1970), Chapter IX: "Fallacies of False Analogy". Fischer does not discuss this particular fallacy, but has the most detailed discussion of analogical arguments and their fallacies of which I am aware.