Symbolism, Imagery, Allegory
The island's giant snakes are a threat—one almost kills Sosie early in the book—but they're also mistreated. After stabbing a snake to death, Sosie and Jennifer discover that it is covered in tumors. "It had probably once been a glorious creature, and Jen was reminded of the old, tough-as-algebra barflies in her neighborhood, the ones with the long, permed hair who still clung to the leopard-print dresses they'd put on thirty years ago and refused to retire." (7.52)
Scary, but sort of sad, too. They even start to feel bad for the snake, even though it tried to kill them. The barflies may not be so dangerous, but same gist.
The snake is an early, foreboding warning of The Corporation's influence on the island. It's later revealed that The Corporation tested many of their products on animals, snakes included. That's why it's so satisfying when a snake eats Harris—it's like the island is getting back at The Corporation. With his stupid swagger and disregard for other people's lives it's only fitting that a snake swallows him, following a hiss that "was deep and personal and very, very pissed off." (38.31) That's a lot of emotion in a snake hiss, yo.
Did you notice that the snake doesn't try to eat the girls who are standing with him? In this scene, the snake's not dining for fun, only for payback. Whew.