Here are our Easter Eggs for Undertale on Playstation 4. The most trustworthy items get the most 'thumbs up' from our users and appear nearer the top!
While you’re in Sans and Papyrus’s house, inspect the sink. After the dog jumps out, go through the sink’s door to enter the Dog Shrine. You can donate money to the dog shrine, and it will gain new decorations. You will earn trophies for doing this, but there is no other purpose or benefit.
The scene where Mettaton traps you and sings about your doomed love before sending you to the dungeon is a reference to the famous opera scene in Final Fantasy VI, particularly the song “Aria di Mezzo Carattere” sung by Celes.
Watch the videos below to compare the two scenes.
If you complete the game as a true pacifist and go through the credits without being hit by a single name, you’ll be able to open the mysterious locked door in Snowdin. This will let you enter the developer room, where you can meet the Annoying Dog.
This might also be a nod to the joke Dog Ending of Silent Hill 2.
If you quickly backtrack at several points during the game, you can catch Flowey stalking you.
Undertale contains several hints about W.D. Gaster, the royal scientist before Alphys took over the position. Strange NPCs known as Gaster’s Followers will sometimes appear with cryptic dialogue about Gaster’s disappearance, referencing how his experiments went wrong and wondering if Alphys will meet the same fate.
It is also possible for a secret room to appear in which you meet a see a strange black-and-white figure fans believe is Gaster.
Additionally, the River Person will sometimes say “Beware of the man who speaks in hands,” which is believed to be a reference to Gaster due to an association between Gaster and the Wingdings font.
It is still unknown if more is planned for Gaster in the future, or if it is just meant as a fun secret for fans to discover.