Since Mixels began, many prototypes have been made by LEGO and found online. A prototype is a first, typical or preliminary model of something, especially a machine, from which other forms are developed or copied. This page encompasses all of the prototypes in the theme's history.
LEGO (Hungry) Monsters
Sometime in the past (most likely around 2012), LEGO created a small line of prototype Monsters. Around the same time, Cartoon Network wanted to co-create a theme with LEGO. LEGO and Cartoon Network worked on the Monsters idea and formed Mixels.
Little is known about the actual Monsters models besides their similarity to Mixels. The models appear to have been slightly smaller than Mixels. Several of these sketch models look similar to the Mixels that would be released in their place. The original concept seemed to have up to five to a tribe, with much simpler designs. They still retained the mix-and-match idea of Mixels, which was the driving point to Cartoon Network.
Some of the current Mixel tribes share color schemes with the preliminary Monsters designs, possibly contributing to the colors.
They appear to have had different pieces, such as a 1x1 half-closed eye piece with a colored eyelid and a less detailed Technic ball eye piece.
Confidentally marked, leaked images of Mixels set bag packaging have been found. Differences to the final product can include a slightly different - or the same - animated form of the character, different artwork in the background, or the character in a different pose.
Images of early designs for the set display boxes in stores have also been found. Often times, the colors of the boxes, and thus, the color representing each wave, are different than the released one.
Sources and References