Zarnot used to be Yang's favorite action figure, until he was brought to life by the Lie Fairy in "The Imagination Situation". Due to the exaggerations brought on by Yang and Yin, Zarnot continued to grow in size and stomped his way into the city to exterminate the two as revenge for being insulted. Zarnot was eventually defeated when Yin and Yang started telling the truth, making him shrink back to his original size. From that point onward, Zarnot vowed he would destroy the twin rabbits, no matter what.
Eventually, Zarnot was recruited by the Night Master, but due to his commonplace acts of failure, the Night Master forced Zarnot into a hated toy box. After the Night Master's defeat, Zarnot's status was unknown until "Deja Foo". In "Wonder Tweens Go", he is upgraded by the Lesson into a more powerful form and briefly renames himself "Generitron" and does a bunch of generic bad guy cliches, such as building a doomsday device, laughing uproariously, and shouting his name. It's not yet known if the transformation is permanent, however, considering the changes the other villains have gone through, it is likely.
It has been revealed in "Dummy Up" that Zarnot still desires Yang's affection and attention, and seems to grow insane jealousies towards any other toy that Yang might be playing with at the time. Whenever Yang ignores him, he seems to want Yang's attention more, and is willing to go far lengths to attain it.