Symbolism, Imagery, Allegory
You can't go a page without finding an animal reference in A Modest Proposal. Horses, cows, deer, and pigs make frequent appearances, mostly so the narrator can compare them to the Irish children. What does that say about the value of human life in Ireland?
Let's take a look at some occasions when animals pop up.
I do therefore humbly offer it to public consideration that of the hundred and twenty thousand children already computed, twenty thousand may be reserved for breed, whereof only one fourth part to be males, which is more than we allow to sheep, black cattle, or swine. (11)
Now, that's a considerate way to treat human children. Those who can't afford to live large are reduced to their market value alone. Also, it's not enough to just slaughter and eat Ireland's babies. Let's earmark a bunch of them as future breeders to add a touch of class.
By the end of the essay, notice how the narrator subs out the word "children" for "mortals"—as in, ask the parents of these mortals how they feel about their kids being eaten (34). No one says "Aw, cute. Look at that mortal." It's a word that strips away any sympathetic portrayal and leaves you with the sense that humans are becoming the animals.