Prior to Undertale 's release, Toby Fox worked on a variety of projects, ranging from one-off compositions, to fan works of other media, to contributing to the Homestuck Original Soundtrack.
Interestingly, some of Toby Fox's prior compositions are either highly reminiscent of or nearly identical to some motifs used in Undertale and Deltarune.
Additionally, some songs and motifs in Undertale and Deltarune are evidently inspired by others' work.
This article will cover a variety of examples of this, ranging from simple inspiration, to the re-use of particular motifs.
Certain songs in Undertale and Deltarune were initially made for prior projects, only to be repurposed.
From the Undertale sheet music booklet:
"Ironically, this is one of many songs in this booklet not to be originally composed for Undertale."
"I actually created this for a fan-album called "I Miss You" as an attempt at creating an original "Game Over" theme."
Indeed, Fallen Down, the theme of Toriel, was orginally composed for the 2012 Earthbound tribute album I Miss You, as Fallen Down.
(See also: 'Fallen Down')
Also from the Undertale sheet music booklet, under Nyeh Heh Heh! :
"This song wasn't made for Undertale either."
"It (and Bonestrousle) were the main battle themes of an RPG game I was working on before Undertale, based off of a dream I had."
"However, I didn't plan it very well, so the project fell apart before I accomplished a single room."
" "Sans" was also a song in the game called "Muscles," and "Heartache" was a song called "Joker Battle." "
If you're familiar with the origins of Deltarune, this anecdote may ring a bell.
However, all that's relevant here is that these four were originally created prior to Undertale, and were simply repurposed for the game.
(See also: 'Nyeh Heh Heh!', 'sans.', and 'Heartache')
Sometimes, as opposed to the reuse of a full song, Toby Fox instead references only a part of a previous track.
Patient, composed by Toby Fox, is a remix of Doctor, the background music for the Land of Wind and Shade from Homestuck.
There are a few things of note about this:
('Another Medium (Ostinato)', as heard in Waterfall and Another Medium, respectively)
Patient : 0:29-0:59
Of course, that's not the only similarity: both Another Medium and Patient use the exact same instrumenetation, including the same drum samples used in Cave Story.
(See also: 'Another Medium')
Cucumber Quest, written by Gigi D.G., is a web comic 'about bunny kids going on adventures', and is (as of writing) on indefinite hiatus.
Gigi D.G. is also credited for various pieces of concept art for Deltarune.
Importantly, Toby Fox composed two fan-made songs for the characters Noisemaster and Nightmare Knight, which appear to be the origins of two motifs:
('Metal Crusher', as heard in Metal Crusher)
Noisemaster : 0:35-0:52, 1:28-1:44
('The Door', as heard in The Chase)
Nightmare Knight : 0:48-1:11
Later on in the song, the melody is played in double-time, and slightly remixed:
Nightmare Knight : 1:30-1:53
(See also: 'Metal Crusher' and 'The Door')
(Web archive of Noisemaster)
(Web archive of Nightmare Knight)
Penumbra Phantasm was an unreleased Homestuck track composed by Toby Fox.
Although the song itself never saw the light of day, some argue that its influence (and perhaps, leitmotifs) can be heard in not only Undertale, but Deltarune as well.
In fact, it's a big enough topic that I decided to put give it its own page:
On Penumbra Phantasm
Without delving too deeply into the specifics, here are the parts that are supposedly connected.
Hopes and Dreams, 1:18-1:29 and Penumbra Phantasm, 0:43-1:05:
(Timestamps 1:18-1:29 of Hopes and Dreams )
Penumbra Phantasm : 0:43-1:05
And 'THE HOLY B' and Penumbra Phantasm, 1:05-1:31:
('THE HOLY B', as heard in THE HOLY )
Penumbra Phantasm : 1:05-1:31
(See also: 'THE HOLY' and On Penumbra Phantasm)
(Youtube upload of Penumbra Phantasm)
A bit of an odd case, treasure_discovered.mp3 is a short track uploaded to Toby Fox's website, which features a backing melody later reused for THE HOLY :
('THE HOLY (BACKING)', as heard in THE HOLY )
treasure_discovered.mp3 : 0:00-0:41
(Web archive of treasure_discovered.mp3)
Here, we have an example of a leitmotif from another project being used in Undertale.
Item Haver is (was?) a game in development by VideoChess who, like Gigi D.G., later became a member of the Deltarune team.
I found it particularly difficult to find information regarding the game online, due to a combination of not having a Twitter account and having to navigate a maze of broken links and deactivated accounts across multiple sites.
That is to say, I can't really say much how the music in question was to actually be used.
Regardless, Toby Fox's website at one point hosted two tracks presumed to have been made for the game, havercity.mp3 and mystery.mp3 :
(havercity.mp3 and mystery.mp3, respectively)
The leitmotif these two share was later referenced in Undertale via the 'ballchime' easter egg; by typing B A L L on the keyboard on the title screen, a sound will play:
(snd_ballchime.wav)
This 'ballchime' appears to be the first three seconds of the melody of mystery.mp3.