Once Upon A Truthful Time is the 1st track in the Undertale Yellow Soundtrack. It will play during the opening introduction when starting the game. This song was also used in the official trailer for the game.

As the name implies, the song is a remix of Once Upon a Time, only with added western notes.

Undertale Yellow Soundtrack
1-10 Once Upon a Truthful Time • Main Menu 01 • Enemy Approaching Yellow • Fallen Down Yellow
Ruins Yellow • Howdy! • In Darkness • Main Menu 02 • Justice • Fever Pitch! • Enemy Retreating • Seclusion • Forlorn • House Guest
10-16 Enemy Retreating • Seclusion • Forlorn • House Guest • To Be Continued • Fun-Sized
Bonus Boppin'