The Egg is a secret Key Item. It can be acquired from a man in the hidden area in Forest that the party may randomly encounter, as well as in Chapter 2's Dog Car room from the same man behind the tree.
If acquired, the Egg remains in the party's inventory when returning from the Dark World.
- The Egg can be put into Asgore's refrigerator, after which there are mysteriously two eggs in the refrigerator.
- The Egg can also be put into the eggpile in Sans' store at the end of Chapter 2, after which there will be "more eggs than usual" in the pile.
- The Egg can be dropped, which results in the unique text: "What egg?"
- After speaking with Temmie in the classroom after the end of Chapter 2, she places an egg on the table. Interacting with this egg displays the number of eggs you have found in the game, regardless of what you have done with them.
- Unlike other items in the party's inventory, the Egg does not disappear or change into something else after Kris leaves the Dark World.
- The text that appears when the egg is disposed of is likely a reference to the recurring phrase "What pumpkin?," a running gag throughout the MS Paint Adventures webcomic series.
- The Egg is possibly referencing Easter eggs, which are hidden objects or references within a game.
- The Egg may be a reference to or appearance of "Mystery Man", a secret character in Undertale found in a similar fashion.
- It is possible that The Egg may have vague connections to The Knight, since Egg spelled in wingdings is ☜︎☝︎☝︎, similar to how a knight moves in a game of chess.