The boar makes a couple of appearances in Part 3 of our book, but we think it packs the biggest symbolic punch when Hassan and Mallory are hunting in the Alps. There are two rules for the hunting trip: No killing babies, and whatever is caught will be shared by everyone.
Perhaps unsurprisingly, Mallory breaks the first rule and kills a young boar. When she does, we can understand the boar as a symbol for both Mallory's disregard for the desires of others (which she shows by eschewing the rules) and her icy heart. After all, killing a baby animal isn't exactly the work of a softy.
Initially, Mallory doesn't seem remotely repentant—she even goes so far as to say the baby boar flesh is "'so tasty to eat'" (8.146)—but later, when Mallory is stuck in the chapel, she envisions a boar's head on the table at the Last Supper. This head stares at her, and as it does, we understand that she is considering her life and the decisions she's made differently than she has before. Mallory is changing, and the reappearance of the boar let's us know that her callous disregard for the lives of others is on its way out. In the boar's face she sees:
[…] the balance sheet of her life, an endless list of credits and debits, of accomplishments and failures, small acts of kindness and real acts of cruelty. (10.52)
The boar looks into her soul and its eyes reflect all the terrible things she's done. All she can do is look back without arguing and just deal with it. It is a rebirth of sorts for Mallory, which is fitting since in Hinduism, the boar is closely associated with creation stories. The boar is a symbol for Mallory's own re-creation.