Collection: Undertale

In 2019, a YouTube user by the name of MickeySerbia posted a now-deleted video showing that Sans' voice grunting was most likely sourced from a snippet of Patrick Star saying "maybe it's the way you're dressed" in the SpongeBob SquarePants episode "Something Smells"; specifically, the grunt comes from the "ay" in "maybe." This information was further popularized when the SoundCloud account UNDERTALE OST: Recreated made and published the same discovery later that year, independent of MickeySerbia. While Toby Fox did not comment on the discovery, waveform comparisons between the SoundCloud post and Sans' voice grunting shows that the two are nearly identical apart from a slight delay in the former.

According to Toby Fox, Alphys' design in both Undertale and Deltarune was partly inspired by John Egbert from the webcomic Homestuck, which Fox had previously composed music for.