Some believe "QuackPrep" was an internal developer room—specifically a preparation room for testing quack sounds (the rubber chicken noise Alphys makes) or duck enemies. In this theory, "Prep" is short for "Preparation," and the room was deleted for being too unstable.
Playing QuackPrep Undertale offers several benefits, including: quackprep undertale
But does that matter? In the Undertale fandom, the line between official and fanon has always been blurry. When thousands of people believe in a secret duck, share art of it, and write music for its boss fight, that secret becomes real in the only place that matters: the collective imagination. "Prep" is short for "Preparation