Eggs are collectible items that first appeared in version 0.100h. There are currently ten eggs and counting in the game, placed throughout specific maps. Each egg gives you a new menu skin.
Forest Pier
You can find one egg here. This egg is in the Southern Area, around the giant half-buried sun statue.
Depths
You can find two eggs here. One is on the Aquarium and the another is in the Deepest part of the Depths.
Hidden Shoal
You can find one egg here.
Underground
You can find one egg here.
Planetarium
There's one egg in the scenery area. You can find this place in the Deserted Town.
Factory Garden
There's one egg in the bottom to the map, you only need get off all the stairs in the Industrial Maze. You can reach this place via Arc De Pillar World
Neon Aquatic Cavern
There's one egg in this place. This place can be reach going to the Factory Garden, find the Aquarium building in the Overgrown City, pass the Victorian Drains, and finally, getting into the boat, find that egg in the Neon Aquatic Cavern.
Guts World
Going through the room with the clown in the grey building leads to a small room with a pumpkin "egg" and much other stuff in it. It will give you a menu skin like the other eggs, but the pumpkin does not disappear when interacted with; it merely turns into an orange pumpkin that doesn't change its color anymore.
Maple Shrine
At the end of the maze is a small orange and head-like "egg". It does not disappear after obtaining the menu skin.
Saturated Eyeball Zone
Killing this NPC will instantly replace your current menu skin with the new one, but it won't be added to the computer's menu skins and the NPC dies permanently. So you can only get it once and it will be gone if you change your menu skin afterwards.