Their main purpose is to grow from someone lying about something, and they will grow to enormous monsters and cause destruction whenever they like. The only way to stop them is to eventually tell the truth, and the monster will shrink and vanish.

Fibs have three forms. The first form is just a small ball-like shape with spots, similar to Lumpy but with an antenna. They will grow bigger with a body and legs. Once the lie is big enough, they will turn into giants with arms with three fingers.