You can experience Easter Egg of all OS versions of Android.

Gingerbread : Zombie Art
Honeycomb : Android Honeybee
IceCreamSandwich : Nyan Droid
JellyBean : BeanBag
Kitkat : DessertCase
Lollipop : Lollipop Land (Supports Jellybean MR1 and above)
Marshmallow : Marshmallow Land (Supports Jellybean MR1 and above)
Nougat : Neko (Supports Lollipop and above)
Lollipop Marshmallow User : Check widget
Nougat Oreo User : Check quick settings
Oreo : Ocquarium
Pie : Paint? (incompletion) [NEW]