Symbolism, Imagery, Allegory
We're just going to say it: Garden of Eden. Whenever you see a snake—and especially if it's called a serpent—it's very likely a shout-out to Adam and Eve and that pesky little creature that prompted their departure from paradise by encouraging Eve to just take a bite of the freaking fruit already.
The fruit, of course, represents knowledge in the Book of Genesis. And in Genesis, Eve's decision to bite into it is a big no-no. She goes directly against God's orders in doing so, and she and her man have to pay big time for this action—just as Rilian has to pay big time for the bite the serpent takes out of his mother as she naps in the "pleasant glade" (a.k.a. Eden). And guess what? This serpent is green, so be sure to read up on that color elsewhere in this section.
As for the potential comparison between Rilian's mom and the forbidden fruit, well, that serpent never should've bitten her. Though it takes years, the serpent is eventually banished from the land forever (thanks to death)—it may be delayed, but it's still comeuppance.