It is the remains of an ocellated puffer fish, known as Gangatoop (Tetradon cutcutia) in Assam. The puffer fish, a balloon-shaped fish of the family Tetradontidae is able to inflate itself enormously with air like a balloon for protecting themselves from predators, and are found both in freshwater and marine environments. This has different varieties and known differently as puffers, balloonfish, blowfish, blowies, bubblefish, swellfish etc.