Hagfish
The hagfish is an eel-shaped marine life that is one of the only known living animals to have a skull but no vertebral column, even though it does have rudimentary vertebrae. Scientists have often argued about how to classify this unique creature.
The hagfish implores a pretty brutal attack whenever it feels like it is being threatened. The sea creature expels a slimy substance that can clog up the gills of its attackers. The material mixes with water and then expands, causing the enemy fish to choke and then release the hagfish to avoid suffocation. Because of this, fish try to mess with the hagfish.