Puffer fish (image source: christthetruth.wordpress.com)
The puffers’s unique and distinctive natural defenses are a compensation for their particular form of locomotion. Puffers use a combination of pectoral, dorsal, anal, and caudal fins for propulsion that make them highly maneuverable but very slow, and therefore comparatively easy targets for predators. As a defense mechanism, puffers have the ability to inflate rapidly, filling their extremely elastic stomachs with water (or air when outside the water) until they are almost spherical in shape. Thus, a hungry predator stalking the puffers may suddenly find itself facing what seems to be a much larger fish and pause, giving the puffers an opportunity to retreat to safety.
See the video below, it’s pretty funny.
More info: en.wikipedia.org