Wilbur from Charlotte's Web ain't got nothin' on these pigs. They're the first thing mentioned in the whole book, so we have to mention them here: "When Augustus came out on the porch the blue pigs were eating a rattlesnake—not a very big one" (1.1).
Gus keeps them around because he thinks the pigs are intelligent. They're smarter, or luckier, than a lot of the men, since they survive the entire trip. "'Why, they're the first pigs to walk all the way from Texas to Montana,' Augustus said. 'That's quite a feat for a pig'" (93.27).
But then Gus dies, and "[a]t Christmas, hungering for pork, they killed Gus's pigs" (100.4). It doesn't matter what your accomplishments are if someone is really hungry for food. That's just the circle of life, we guess. Even Gus had to succumb.