Unreleased bosses are bosses that you can't find in the game, but their textures can be found in the game files.
The Snowman Head
The Emoji
The Fidget Spinner
The Movie Reel
The Turtle
The Watermelon
The Subwoofer
The Tamagochi
The Sunflower
The Barrel
The Pizza
The Clock
The Pinwheel
The Dartboard
The Golden Skull
The Pumpkin