True Pacifist Ending Credits

Upon completing the True Pacifist Route, the player will be subject to credits featuring all of the enemies of the game, and then a bullet hell section in which you must dodge the names of all the kickstarter backers. If you successfully dodge all of the names, you will be able to access the mysterious door in Snowdin Forest. The description of each enemy in the game will be yellow depending on whether or not you took certain actions while in battle with each enemy.

Full Yellow Credit Example
Here is a link to video of someone speedrunning Undertale while achieving all yellow credits.

Trivia

 * Jerry, Glyde and So Sorry do not appear in the credits, nor does Chilldrake or enemies exclusive to Hard mode.
 * Burgerpants is the only vendor that's in the credits, hiding behind Mettaton in a bush.