According to creator and director Toby Fox, the idea for Deltarune originated in 2011, when he had a literal fever dream about witnessing the ending to a game that didn't exist; upon waking up, he became determined to make that game a reality.

Development originally started the following year, but was put on hold before designing even a single room before the project resumed after Undertale's release; two of the songs originally composed during that initial stage would later be reused in Undertale as "Bonetrousle" (previously the main battle theme before being replaced with "Rude Buster") and "Heartache" (previously titled "Joker Battle", apparently being Spoiler:Jevil's early battle theme before being replaced with "THE WORLD REVOLVING").
Contributed by game4brains