The harbour porpoise is a small marine mammal belonging to the family Phocoenidae. It is also commonly referred to by some other names such as the common porpoise, littoral porpoise, and puffing pig. In some countries, it is named as the mereswine, herring hog, and squaletto. These porpoises prefer shallow coastal waters and estuaries and are frequently seen in the North Atlantic and North Pacific Oceans. They are known for their distinctive dark coloration and rounded fins, and are sometimes mistaken for small dolphins. While they may have different names in various regions, all of these synonyms refer to the same fascinating and intelligent species.