When a familiar reaches a certain level, it can be made to metamorphose into a completely new form through the use of a special item.
Go to your Creature cage and familiars with the word "Metamorphosis" beside their name can be fed a special treat to make them metamorphose. The symbol of the familiar (sun, moon, plant, star) has to match the symbol of the drop treat.
Do that with 10 familiars and this will unlock.