Fleens are little green creatures. They're actually an offshoot of the Zoombinis, who left Zoombini society years ago and became mutated by bad hair products. They also became severely harsh and easy to aggravate. This caused their extreme dislike of zoombinis. They can usually be seen in thick forests or dark caves. The red hairs were voiced by Carlos Alazraqui, the blue hairs were voiced by Kath Soucie, the purple hairs were voiced by Scott McNeil, the yellow hairs were voiced by Tress MacNeille, and the green hairs were voiced by Rick Jones.
The Fleens evolved from the Zoombinis and left their civilization, and settled in the Deep, Dark Forest and the eastern part of The Big, the Bad, and the Hungry. The Zoombinis first confronted the Fleens on their way to Zoombiniville, escaping from Zoombini Isle. Each Fleen would choose a particular Zoombini based on his or her characteristics and would chase him or her up a nearby tree. The Zoombinis were able to outsmart them by figuring out the pattern by which the Fleens were choosing Zoombinis to chase and using it against them. (Logical Journey of the Zoombinis)