This is Snakefood, a feeder rat. Snakefood did nothing wrong. She lived an innocent life mingling with her siblings in the window of the fancy rat enclosure when she was plucked from her wood shavings and shoved into a little snakefood box. My idea is that in a stroke of good luck, the family's pet snake gets itself lost, and that the family gets stuck with a rat. Boy is initially unimpressed with rat but eventually adopts it. Adventure etc.
(Based on a true story. My sister now owns a rat instead of a snake, lol. )