In Deltarune, each chapter has an optional Egg collectible hidden throughout the world. They can be used from the inventory, though nothing will happen - whether they are simply easter eggs or have some undiscovered purpose in future is unclear, but here's is how you find every single one in every Chapter!
If you're at Chapter 5, make sure to check out these guides too:
Past the spinning pawn obstacle the loading zone shown below has a 1/50 chance to teleport you to the Egg Room, you'll just have to keep entering the loading zone repeatedly.
After accepting the Egg, in the light world you can go into King flowers and put the egg in Asgore’s Fridge.
Chapter 2 Egg Room
Teleport to Trash Zone and head right until you at the red and yellow car crossings as shown below. This can also be done while Noelle is following Kris during the story.
Hit both the 5 and 9 stop lights and quickly head left then down.
Below the yellow car crossing to the left is a dumpster room with a Green Tree, same as chapter one it has a 1/50 chance to teleport you to the Egg Room when re-entered. In the light world the Egg can be put in Sans’s Shop Egg Basket.
Chapter 3 Egg Room
This Egg room can only be accessed if you have not Lost the Zapper or Ribbick yet ("Lost" meaning that you've reduced their HP to zero in battle by fighting).
Get the Lancer Controller north east of the TV World entrance shortcut, its past the room with many Shadowguys.
After heading down an area take the exit south west into the Lancer Digging Puzzle room, the one shown below.
After turning all the Xs to Os, throw the box to this specific spot between the bushes with Susie, do not solve the box puzzle. You can hit the bell to retry if you missed the box throw.
Next all the way at Green room head downwards and right and get caught by this spotlight. This spotlight shown below won't be here if you’ve Lost the Zapper. You’ll be asked if you're a cheater, say yes, then select Nowhere.
Inside Nowhere solve the box puzzle with Kris and head right, repeatedly talk to Bibliox in the game until dialogue repeats, head right then left, claim your travel ticket from the drawer and you can exit no where by talking to the zapper to the left.
Lastly you'll have to fight a Ribbick, teleport to Goulden Sam and head right. The trash cans in this area should contain a Ribbick encounter. There is also a Ribbick further to the right in another trash can room. In the fight, hover over CroakOnX and hold left. The heart should visibly walk off screen before you take your turn. Once in Mancountry, the Forgotten Man is north of the entrance, to get the Egg you must repeatedly tell him you won't forget him until given an Egg.
The Egg from Chapter 3 cannot be placed. At the beginning of Ch4, it’ll show up on berdly’s desk in the school alongside any other eggs collected before Ch4 if unplaced.
Chapter 4 Egg Room
Inside the 2nd Sanctuary there is a corridor with repeating npcs to the right of the top of the first climbing section. If missed during the 2nd Sanctuary it can be accessed by taking the warp flame to the very beginning then traveling upwards past the room with many Bibliox. There'll be an invisible glass path parallel and above the right exit that leads to a hidden room. Walk all the way left and talk to the Philosopher, answer no. A doorway to the 2nd Sanctuary hallway will open.
Once in the hallway head past the 4th Philosopher until you see an Egg in the glass paned window as shown in the image below, then walk right under it. The hidden room here once again must be re-entered multiple times to access the egg room, but this time it's guaranteed on the 8th re-entry.
In the egg room enter the hospital door with the tree symbol. In the dark will be a canvas, draw a sketch on it. Once the interaction is over Kris will pick up the Egg.
The Chapter 4 Egg does not have a place to be put yet.
Chapter 5 Egg Room
To the right of the Cliffs Shop is a field, cut all of the grass between the gates until you see a blue balloon fly out. One piece of grass will keep growing, keep repeatedly cutting it. Follow this balloon off screen to the Egg Room.
Inside the room interact with the flowers in the same order as the annoying dog to unlock the watering can and water your way to the left. When talking to the Forgotten Man, accept his offering to get the Egg. There is currently no place to put this egg.